avutil/utils: run ff_check_pixfmt_descriptors() only when ASSERT_LEVEL is > 0

this safes about 0.1ms
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/utils.c

@@ -51,7 +51,9 @@ unsigned avutil_version(void)
         av_log(NULL, AV_LOG_ERROR, "Libavutil has been linked to a broken llrint()\n");
     }
 
+#if defined(ASSERT_LEVEL) && ASSERT_LEVEL > 0
     ff_check_pixfmt_descriptors();
+#endif
     checks_done = 1;
     return LIBAVUTIL_VERSION_INT;
 }