Fix res_agi compilation after the const-ify the world merge.

Since we are dealing with a 'const char * const' now, we have to create a
temporary copy of the string to work on rather than the original.  Fix inspired
by reporter.  Reviewed by everyone-and-their-mother in #asterisk-dev.

(closes issue #15184)
Reported by: andrew


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@196270 65c4cc65-6c06-0410-ace0-fbb531ad65f3