avstring: fix return discards qualifiers from pointer target type.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/avstring.c

@@ -51,11 +51,11 @@ int av_stristart(const char *str, const char *pfx, const char **ptr)
 char *av_stristr(const char *s1, const char *s2)
 {
     if (!*s2)
-        return s1;
+        return (char*)(intptr_t)s1;
 
     do {
         if (av_stristart(s1, s2, NULL))
-            return s1;
+            return (char*)(intptr_t)s1;
     } while (*s1++);
 
     return NULL;