check ast_strlen_zero() before calling ast_strdupa() in sip_uri_headers_cmp()

check ast_strlen_zero() before calling ast_strdupa() in sip_uri_headers_cmp()
and sip_uri_params_cmp()

The reporter didn't actually upload a properly-formed patch, instead a
modified chan_sip.c file was uploaded.  I created a patch to determine the
changes, then modified the suggested changes to create a proper fix.  The
summary above is a complete description of the changes.

(closes issue #13547)
Reported by: tecnoxarxa
Patches:
      chan_sip.c.gz uploaded by tecnoxarxa (license 258)
Tested by: tecnoxarxa

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@174082 65c4cc65-6c06-0410-ace0-fbb531ad65f3