Error handling fix in interfaces/libpq/fe-secure.c

Here's another one similar to what I described in my previous message. In
libpq's pqsecure_read(), if SSL_read() returns -1 and sets an error of
SSL_ERROR_SYSCALL, errno may be polluted by libpq_gettext() before a
human-readable string is derived from it. Also, pqReadData() will see the
wrong errno value after the call.

The attached patch fixes both by introducing a named variable to hold the
significant value of errno.

Jeroen