Fix the logic to access the location of a string to free when

setting a new value for a string with av_set_string2().
Fix a segmentation fault.
M    opt.c


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@14219 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/opt.c

@@ -196,7 +196,7 @@ const AVOption *av_set_string2(void *obj, const char *name, const char *val, int
     }
 
     if(alloc){
-        av_free((void*)(((uint8_t*)obj) + o->offset));
+        av_free(*(void**)(((uint8_t*)obj) + o->offset));
         val= av_strdup(val);
     }