NFSv4: check return value of xdr_inline_decode

Function xdr_inline_decode() will return a NULL pointer if the input
buffer does not have long enough buffer to decode nbytes of data.
However, in function decode_op_map(), the return value of
xdr_inline_decode() is not validated before it is used. This patch adds
a check to the return value of xdr_inline_decode().
Signed-off-by: NPan Bian <bianpan2016@163.com>
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/nfs4xdr.c

@@ -5570,6 +5570,8 @@ static int decode_op_map(struct xdr_stream *xdr, struct nfs4_op_map *op_map)
 	unsigned int i;
 
 	p = xdr_inline_decode(xdr, 4);
+	if (!p)
+		return -EIO;
 	bitmap_words = be32_to_cpup(p++);
 	if (bitmap_words > NFS4_OP_MAP_NUM_WORDS)
 		return -EIO;