SUNRPC: svcauth_gss: avoid NULL deref on zero length gss_token in gss_read_proxy_verf

A zero length gss_token results in pages == 0 and in_token->pages[0]
is NULL. The code unconditionally evaluates
page_address(in_token->pages[0]) for the initial memcpy, which can
dereference NULL even when the copy length is 0. Guard the first
memcpy so it only runs when length > 0.

Fixes: 5866efa8cbfb ("SUNRPC: Fix svcauth_gss_proxy_init()")
Cc: [e-mail]
Signed-off-by: Joshua Rogers <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

net/sunrpc/auth_gss/svcauth_gss.c

···
 	}
 
 	length = min_t(unsigned int, inlen, (char *)xdr->end - (char *)xdr->p);
-	memcpy(page_address(in_token->pages[0]), xdr->p, length);
+	if (length)
+		memcpy(page_address(in_token->pages[0]), xdr->p, length);
 	inlen -= length;
 
 	to_offs = length;