Prevent crash in 'sip show peers' when the number of peers on a system is large

When you have lots of SIP peers (according to the issue reporter, around 3500),
the 'sip show peers' CLI command or AMI action can crash due to a poorly placed
string duplication that occurs on the stack. This patch refactors the command
to not allocate the string on the stack, and handles the formatting of a single
peer in a separate function call.

(closes issue ASTERISK-21466)
Reported by: Guillaume Knispel
patches:
  fix_sip_show_peers_stack_overflow_asterisk_11.3.0-v2.patch uploaded by gknispel (License 6492)
........

Merged revisions 387134 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@387135 f38db490-d61c-443f-a65b-d21fe96a405b