Fix compiler warnings: clobber variables

This patchs fixes warnings generated by older toolchains:

shared/image-loader.c: In function 'load_png':
shared/image-loader.c:211:12: warning: variable 'data' might be clobbered by 'longjmp' or 'vfork' [-Wclobbered]
  png_byte *data = NULL;

See https://linux.die.net/man/3/longjmp why is this needed.

Signed-off-by: Marius Vlad <marius.vlad0@gmail.com>

shared/image-loader.c

@@ -208,8 +208,8 @@ load_png(FILE *fp)
 {
 	png_struct *png;
 	png_info *info;
-	png_byte *data = NULL;
-	png_byte **row_pointers = NULL;
+	png_byte *volatile data = NULL;
+	png_byte **volatile row_pointers = NULL;
 	png_uint_32 width, height;
 	int depth, color_type, interlace, stride;
 	unsigned int i;