Consistently check the fd value

Follow the approach used through the rest of the project.

Suggested-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>

intel/test_decode.c

@@ -56,7 +56,7 @@ read_file(const char *filename, void **ptr, size_t *size)
 	struct stat st;
 
 	fd = open(filename, O_RDONLY);
-	if (fd == -1)
+	if (fd < 0)
 		errx(1, "couldn't open `%s'", filename);
 
 	ret = fstat(fd, &st);

tests/name_from_fd.c

@@ -45,7 +45,7 @@ int main(int argc, char **argv)
 	char *v;
 
 	fd = open("/dev/dri/card0", O_RDWR);
-	if (fd == -1)
+	if (fd < 0)
 		return 0;
 
 	v = drmGetDeviceNameFromFd(fd);