libcamera: yaml_parser: Fix uninitialized variable warning

gcc 14.3.0, cross-compiling from amd64 to arm64, warns about a possibly
uninitialized variable in YamlObject::Getter<>::get():

In file included from ../../include/libcamera/internal/yaml_parser.h:12,
                 from ../../src/libcamera/yaml_parser.cpp:8:
In constructor ‘constexpr std::_Optional_payload_base<_Tp>::_Storage<_Up, <anonymous> >::_Storage(std::in_place_t, _Args&& ...) [with _Args = {unsigned char}; _Up = unsigned char; bool <anonymous> = true; _Tp = unsigned char]’,
    inlined from ‘constexpr std::_Optional_payload_base<_Tp>::_Optional_payload_base(std::in_place_t, _Args&& ...) [with _Args = {unsigned char}; _Tp = unsigned char]’ at include/c++/14.3.0/optional:122:4,
    inlined from ‘constexpr std::_Optional_payload<unsigned char, true, true, true>::_Optional_payload(std::in_place_t, _Args&& ...) [with _Args = {unsigned char}][inherited from std::_Optional_payload_base<unsigned char>]’ at include/c++/14.3.0/optional:337:42,
    inlined from ‘constexpr std::_Optional_base<_Tp, true, true>::_Optional_base(std::in_place_t, _Args&& ...) [with _Args = {unsigned char}; typename std::enable_if<is_constructible_v<_Tp, _Args ...>, bool>::type <anonymous> = false; _Tp = unsigned char]’ at include/c++/14.3.0/optional:648:4,
    inlined from ‘constexpr std::optional<_Tp>::optional(_Up&&) [with _Up = unsigned char; typename std::enable_if<__and_v<std::__not_<std::is_same<std::optional<_Tp>, typename std::remove_cv<typename std::remove_reference<_Up>::type>::type> >, std::__not_<std::is_same<std::in_place_t, typename std::remove_cv<typename std::remove_reference<_Up>::type>::type> >, std::is_constructible<_T1, _U1>, std::is_convertible<_Iter, _Iterator> >, bool>::type <anonymous> = true; _Tp = unsigned char]’ at include/c++/14.3.0/optional:747:47,
    inlined from ‘std::optional<_Tp> libcamera::YamlObject::Getter<T, typename std::enable_if<(((((is_same_v<signed char, T> || is_same_v<unsigned char, T>) || is_same_v<short int, T>) || is_same_v<short unsigned int, T>) || is_same_v<int, T>) || is_same_v<unsigned int, T>), void>::type>::get(const libcamera::YamlObject&) const [with T = unsigned char]’ at ../../src/libcamera/yaml_parser.cpp:172:10:
include/c++/14.3.0/optional:210:15: error: ‘value’ may be used uninitialized [-Werror=maybe-uninitialized]
  210 |             : _M_value(std::forward<_Args>(__args)...)
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../src/libcamera/yaml_parser.cpp: In member function ‘std::optional<_Tp> libcamera::YamlObject::Getter<T, typename std::enable_if<(((((is_same_v<signed char, T> || is_same_v<unsigned char, T>) || is_same_v<short int, T>) || is_same_v<short unsigned int, T>) || is_same_v<int, T>) || is_same_v<unsigned int, T>), void>::type>::get(const libcamera::YamlObject&) const [with T = unsigned char]’:
../../src/libcamera/yaml_parser.cpp:165:19: note: ‘value’ was declared here
  165 |                 T value;
      |                   ^~~~~

This appears to be a false positive, as the std::from_chars() function
should set value when it returns without an error. Commit bb1d216113
("libcamera: base: log: Fix uninitialized variable warning") fixed a
similar warning with gcc 13.3.0.

The warning is easy to work around by initializing the variable, and
doing so shouldn't be too costly as the type T is restricted to being an
integer. Fix the build by doing so.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>

src/libcamera/yaml_parser.cpp

@@ -162,7 +162,7 @@ struct YamlObject::Getter<T, std::enable_if_t<
 			return std::nullopt;
 
 		const std::string &str = obj.value_;
-		T value;
+		T value = {};
 
 		auto [ptr, ec] = std::from_chars(str.data(), str.data() + str.size(),
 						 value);