clang compiler warnings: Fix warning for -Wgnu-variable-sized-type-not-at-end

This patch fixes a warning caught by clang, wherein a variable sized struct is
not located at the end of a struct. While the code in question actually
expected this, this is a good warning to watch for. Hence, this patch refactors
the code in question to not have two variable length elements in the same
struct.

Review: https://reviewboard.asterisk.org/r/4530/

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4530.patch submitted by dkdegroot (License 6600)
........

Merged revisions 433717 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 433718 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433719 65c4cc65-6c06-0410-ace0-fbb531ad65f3