channels/chan_mgcp: Fix regression which causes gateways to be skipped

channels/chan_mgcp: Fix regression which causes gateways to be skipped

In r227276, a while loop was turned into a for loop. Unfortunately, a portion
of the while loop was left in the code such that, when a static gateway is
encountered in the list of MGCP gateways, the next gateway would be skipped.
At best, we would simply flip past a gateway; at worst, this could lead to a
crash.

ASTERISK-24500 #close
Reported by: Xavier Hienne
patches:
  chan_mgcp.patch uploaded by Xavier Hienne (License 6657)
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Merged revisions 427613 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 427614 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@427615 65c4cc65-6c06-0410-ace0-fbb531ad65f3