dshow:protect access to curbufsize to promote consistency

Signed-off-by: rogerdpack <rogerpack2005@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/dshow.c

@@ -207,11 +207,11 @@ callback(void *priv_data, int index, uint8_t *buf, int buf_size, int64_t time)
 
 //    dump_videohdr(s, vdhdr);
 
-    if(shall_we_drop(s))
-        return;
-
     WaitForSingleObject(ctx->mutex, INFINITE);
 
+    if(shall_we_drop(s))
+        goto fail;
+
     pktl_next = av_mallocz(sizeof(AVPacketList));
     if(!pktl_next)
         goto fail;