Fix ppp_deflate issues with recent zlib_inflate changes

The last zlib_inflate update broke certain corner cases for ppp_deflate
decompression handling.  This patch fixes some logic to make things work
properly again.  Users other than ppp_deflate (the only Z_PACKET_FLUSH
user) should be unaffected.

Fixes bug 8405 (confirmed by Stefan)
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Cc: Stefan Wenk <stefan.wenk@gmx.at>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/zlib_inflate/inflate.c

@@ -743,12 +743,14 @@ int zlib_inflate(z_streamp strm, int flush)
 
     strm->data_type = state->bits + (state->last ? 64 : 0) +
                       (state->mode == TYPE ? 128 : 0);
-    if (((in == 0 && out == 0) || flush == Z_FINISH) && ret == Z_OK)
-        ret = Z_BUF_ERROR;
 
     if (flush == Z_PACKET_FLUSH && ret == Z_OK &&
-            (strm->avail_out != 0 || strm->avail_in == 0))
+            strm->avail_out != 0 && strm->avail_in == 0)
 		return zlib_inflateSyncPacket(strm);
+
+    if (((in == 0 && out == 0) || flush == Z_FINISH) && ret == Z_OK)
+        ret = Z_BUF_ERROR;
+
     return ret;
 }