Add coverity +kill annotation to scripting.c traceback()

Coverity was showing a number of false positives because it did not
understand that in the context of scripting.c calling ScriptError()
effectively terminated the calling routine, just like a return or
an abort().

Coverity would report uninitialized variables or out-of-bounds reads
or writes and the like, as it analysed code following a call to
ScriptError().  For example, in fontforge/scripting.c :: bSqrt() it
would report that variable 'val' could be uninitialized in the call
to sqrt().

There is a mechanism to instruct Coverity that a routine should be
considered as terminating processing.  Adding a "function annotation"
before the function gives it the information that should prevent it
from being so confused.

This change adds such an annotation to routine traceback(), which is
the actual routine that uses the quite rude longjmp() to terminate
normal processing.
>         /* coverity[+kill] */
>         static void traceback(Context *c) {

We should inspect the _next_ Coverity report and see if this clears
up a number of the false reports.  If not it may be that this same
function annotation needs to be added to all the _using_ routines,
such as ScriptError().
Merge pull request #1656 from tshinnic/coverity_kill_script_traceback

Add coverity +kill annotation to scripting.c traceback()