parseutils: fix const removal warning

The const qualifier is still removed although it happens inside
the strtol() function so no warning is generated.

Fixes:
libavutil/parseutils.c:110:11: warning: assignment discards qualifiers from pointer target type

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavutil/parseutils.c

@@ -107,8 +107,7 @@ int av_parse_video_size(int *width_ptr, int *height_ptr, const char *str)
         }
     }
     if (i == n) {
-        p = str;
-        width = strtol(p, &p, 10);
+        width = strtol(str, &p, 10);
         if (*p)
             p++;
         height = strtol(p, &p, 10);