make av_strdup(NULL) return NULL

Originally committed as revision 13250 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/mem.c

@@ -139,11 +139,13 @@ void *av_mallocz(unsigned int size)
 
 char *av_strdup(const char *s)
 {
-    char *ptr;
+    char *ptr= NULL;
+    if(s){
     int len = strlen(s) + 1;
     ptr = av_malloc(len);
     if (ptr)
         memcpy(ptr, s, len);
+    }
     return ptr;
 }