AST-2016-002 chan_sip.c: Fix retransmission timeout integer overflow.

AST-2016-002 chan_sip.c: Fix retransmission timeout integer overflow.

Setting the sip.conf timert1 value to a value higher than 1245 can cause
an integer overflow and result in large retransmit timeout times.  These
large timeout times hold system file descriptors hostage and can cause the
system to run out of file descriptors.

NOTE: The default sip.conf timert1 value is 500 which does not expose the
vulnerability.

* The overflow is now detected and the previous timeout time is
calculated.

ASTERISK-25397 #close
Reported by: Alexander Traud

Change-Id: Ia7231f2f415af1cbf90b923e001b9219cff46290