libcamera: controls: Initialize ControlInfoMap::idmap_

The compiler generated constructor does not initialize the
ControlInfoMap::idmap_ field.

Fix this by explicitly initializing the field in the class
declaration.

Reported-by: Coverity CID=354657
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/controls.h

@@ -343,7 +343,7 @@ public:
 private:
 	bool validate();
 
-	const ControlIdMap *idmap_;
+	const ControlIdMap *idmap_ = nullptr;
 };
 
 class ControlList