Fix reconnecting in some cases when using MOSQ_OPT_TLS_USE_OS_CERTS.

Fix reconnecting failing when MOSQ_OPT_TLS_USE_OS_CERTS was in use, but none
of capath, cafile, psk, nor MOSQ_OPT_SSL_CTX were set, and
MOSQ_OPT_SSL_CTX_WITH_DEFAULTS was set to the default value of true.

Closes #2288. Thanks to Poltorak Serguei.