fix segmentation fault of lws_snprintf

[zzblydia]

I am working with libwebsockets v4.3.0 as a ws client.
It bears resemblance to #3171, differing in that I don't handle the callback LWS_CALLBACK_CLIENT_APPEND_HANDSHAKE_HEADER. so the null pointer p comes to p += lws_snprintf(p, lws_ptr_diff_size_t(end, p), "\x0d\x0a");

The SIGSEGV stack trace on suse platform is as follows:

#0 vsnprintf (string=0x0, maxlen=93824992762488, ....) at ...
#1 lws_snprintf (str=0x0, size=93824992762488, ...) at libwebsockets.c:762
#2 lws_generate_client_handshake (...) at client-http.c:1296

I have observed numerous instances of the usage of p += lws_snprintf(p, ...).
so I believe it would be prudent to verify the null pointer within the functions lws_snprintf and lws_strncpy.

[lws-team]

Thanks, it's pushed on main.