general: Fix memory Corruption in __ast_string_field_ptr_build_va.

If the space left in a stringfield is between 0 and
(alignof(ast_string_field_allocation)-1) adding new data would cause
memory corruption, because we would assume enough space (unsigned
underrun).

Thanks Arnd Schmitter for reporting and finding out the cause!

ASTERISK-23508 #close
Reported by: Arnd Schmitter
Tested by: Arnd Schmitter, JoshE

Review: https://reviewboard.asterisk.org/r/3898/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@420680 f38db490-d61c-443f-a65b-d21fe96a405b