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

In the Linux kernel, the following vulnerability has been resolved:

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.