jitterbuffer: Correct signed/unsigned mismatch causing assert

If the system time has stepped backwards because of a time
adjustment between the time a frame is timestamped and the
time we check the timestamps in abstract_jb:hook_event_cb(),
we get a negative interval, but we don't check for that there.
abstract_jb:hook_event_cb() then calls
fixedjitterbuffer:fixed_jb_get() (via abstract_jb:jb_get_fixed)
and the first thing that does is assert(interval >= 0).

There are several issues with this...

 * abstract_jb:hook_event_cb() saves the interval in a variable
   named "now" which is confusing in itself.

 * "now" is defined as an unsigned int which converts the negative
   value returned from ast_tvdiff_ms() to a large positive value.

 * fixed_jb_get()'s parameter is defined as a signed int so the
   interval gets converted back to a negative value.

 * fixed_jb_get()'s assert is NOT an ast_assert but a direct define
   that points to the system assert() so it triggers even in
   production mode.

So...

 * hook_event_cb()'s "now" was renamed to "relative_frame_start" and
   changed to an int64_t.
 * hook_event_cb() now checks for a negative value right after
   retrieving both the current and framedata timestamps and just
   returns the frame if the difference is negative.
 * fixed_jb_get()'s local define of ASSERT() was changed to call
   ast_assert() instead of the system assert().

ASTERISK-29480
Reported by: Dan Cropp

Change-Id: Ic469dec73c2edc3ba134cda6721a999a9714f3c9