Clean up use of va_end/va_args in res_config_mysql

There were several problems using variadic argument macros in res_config_mysql.
 * Improper use of va_end. Multiple calls to va_end were possible resulting in
   an unbalanced matching of va_start/va_end.
 * Calls to va_arg after a possible encounter of a SENTINEL value.

This patch corrects those errors.

(closes issue ASTERISK-19451)
Reported by: wdoekes
patches:
  ASTERISK-19451-1.8--2.diff uploaded by wdoekes (License 5674)
........

Merged revisions 382021 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@382022 f38db490-d61c-443f-a65b-d21fe96a405b