Fix bogus error checking on unique_fd

The expression "!fd" calls the implicit conversion to int, but comparing
the raw fd against 0 does not work, since open() and other POSIX calls
returning a file descriptor use -1 to signal an error.

Test: m recovery
Change-Id: I0847c276f39cb9dd09c7ffb96951276113418fc8

applypatch/applypatch.cpp

@@ -76,7 +76,7 @@ static bool ReadPartitionToBuffer(const Partition& partition, FileContents* out,
   }
 
   android::base::unique_fd dev(open(partition.name.c_str(), O_RDONLY));
-  if (!dev) {
+  if (dev == -1) {
     PLOG(ERROR) << "Failed to open eMMC partition \"" << partition << "\"";
   } else {
     std::vector<unsigned char> buffer(partition.size);