Optimize / simplify avcodec_get_pix_fmt_name() check.

Originally committed as revision 25689 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/imgconvert.c

@@ -419,7 +419,7 @@ void avcodec_get_chroma_sub_sample(enum PixelFormat pix_fmt, int *h_shift, int *
 
 const char *avcodec_get_pix_fmt_name(enum PixelFormat pix_fmt)
 {
-    if (pix_fmt < 0 || pix_fmt >= PIX_FMT_NB)
+    if ((unsigned)pix_fmt >= PIX_FMT_NB)
         return NULL;
     else
         return av_pix_fmt_descriptors[pix_fmt].name;