avutil/file: add more check befor destory the buffer

add more check befor destory the buffer
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavutil/file.c

@@ -143,7 +143,7 @@ out:
 
 void av_file_unmap(uint8_t *bufptr, size_t size)
 {
-    if (!size)
+    if (!size || !bufptr)
         return;
 #if HAVE_MMAP
     munmap(bufptr, size);