Prevent overflow in calculation in ast_tvdiff_ms on 32-bit machines

Prevent overflow in calculation in ast_tvdiff_ms on 32-bit machines

The method ast_tvdiff_ms attempts to calculate the difference, in milliseconds,
between two timeval structs, and return the difference in a 64-bit integer.
Unfortunately, it assumes that the long tv_sec/tv_usec members in the timeval
struct are large enough to hold the calculated values before it returns.  On
64-bit machines, this might be the case, as a long may be 64-bits.  On 32-bit
machines, however, a long may be less (32-bits), in which case, the calculation
can overflow.

This overflow caused significant problems in MixMonitor, which uses the method
to determine if an audio factory, which has not presented audio to an audiohook,
is merely late in providing said audio or will never provide audio.  In an
overflow situation, the audiohook would incorrectly determine that an audio
factory that will never provide audio is merely late instead.  This led to
situations where a MixMonitor never recorded any audio.  Note that this happened
most frequently when that MixMonitor was started by the ConfBridge application
itself, or when the MixMonitor was attached to a Local channel.

(issue ASTERISK-19497)
Reported by: Ben Klang
Tested by: Ben Klang
Patches:
  32-bit-time-overflow-10-2012-04-26.diff (license #6283) by mjordan

(closes issue ASTERISK-19727)
Reported by: Mark Murawski
Tested by: Michael L. Young
Patches:
  32-bit-time-overflow-2012-04-27.diff (license #6283) by mjordan)

(closes issue ASTERISK-19471)
Reported by: feyfre
Tested by: feyfre

(issue ASTERISK-19426)
Reported by: Johan Wilfer

Review: https://reviewboard.asterisk.org/r/1889/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@364277 65c4cc65-6c06-0410-ace0-fbb531ad65f3