make av_strdup(NULL) return NULL

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@13250 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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;
 }