BUG/MINOR: checks: email-alert causes a segfault when an unknown mailers section...

BUG/MINOR: checks: email-alert causes a segfault when an unknown mailers section is configured

A segfault can occur during at the initialization phase, when an unknown
"mailers" name is configured. This happens when "email-alert myhostname" is not
set, where a direct pointer to an array is used instead of copying the string,
causing the segfault when haproxy tries to free the memory.

This is a minor issue because the configuration is invalid and a fatal error
will remain, but it should be fixed to prevent reload issues.

Example of minimal configuration to reproduce the bug :
    backend example
        email-alert mailers NOT_FOUND
        email-alert from foo@localhost
        email-alert to bar@localhost

This fix must be backported to 1.6.