chan_sip: Fix handling of "From" headers longer than 256 characters

From headers were processed using a 256 character buffer on the stack.
This change replaces that with a heap allocation by ast_strdup.

ASTERISK-23790 #close
Reported by: uniken1
Tested by: uniken1
Review: https://reviewboard.asterisk.org/r/3669/
Patches:
    chan_sip-large-from-header-1.8-r3.patch uploaded by wdoekes (license 5674)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@417248 f38db490-d61c-443f-a65b-d21fe96a405b