avpacket: Fix error checking in packet_alloc

Previously the wrong buffer pointer was checked, when buf
instead of *buf was checked. But checking the return value
instead is even better.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/avpacket.c

@@ -64,12 +64,13 @@ FF_ENABLE_DEPRECATION_WARNINGS
 
 static int packet_alloc(AVBufferRef **buf, int size)
 {
+    int ret;
     if ((unsigned)size >= (unsigned)size + FF_INPUT_BUFFER_PADDING_SIZE)
         return AVERROR(EINVAL);
 
-    av_buffer_realloc(buf, size + FF_INPUT_BUFFER_PADDING_SIZE);
-    if (!buf)
-        return AVERROR(ENOMEM);
+    ret = av_buffer_realloc(buf, size + FF_INPUT_BUFFER_PADDING_SIZE);
+    if (ret < 0)
+        return ret;
 
     memset((*buf)->data + size, 0, FF_INPUT_BUFFER_PADDING_SIZE);