Change the write-side check to match the read-side and the OpenSSL API

[ValentinChernovNTQ]

File: lib/net_mosq.c, function net__write(): Change the write-side check to match the read-side and the OpenSSL API contract

This is a minimal, symmetric, low-risk change: it makes net__write() handle SSL_write() failures exactly the way net__read() already handles SSL_read() failures, using the same, already-tested net__handle_ssl() path.

Fixes: #3354

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