easy: validate buffer pointer in curl_easy_recv and curl_easy_send

[filtede98]

Summary

Both curl_easy_recv() and curl_easy_send() previously validated !n but did not validate whether buffer is NULL when a non-zero buflen is passed. Calling either function with buflen > 0 and buffer == NULL passed the pointer directly down to network socket or TLS library read/write routines, which can lead to crashes or undefined behavior.

This patch adds (buflen && !buffer) validation to both curl_easy_recv() and curl_easy_send():

• Returns CURLE_BAD_FUNCTION_ARGUMENT if buffer is NULL while buflen > 0.
• Keeps behavior consistent with curl_ws_send() and curl_ws_recv().
• Adds regression assertions in tests/libtest/lib556.c.