autom4te: don’t crash when warnings have no stacktrace

autom4te: don’t crash when warnings have no stacktrace

In testing on Darwin (OSX), sometimes warnings reported from M4 code
reach autom4te with no stack trace at all, causing the perl script to
crash with a “use of uninitialized value” error.  The root cause of
the problem is not clear to me, but the script certainly shouldn’t
crash.

Problem found by Jannick <thirdedition@gmx.net>.

* bin/autom4te.in: When processing warnings, make sure $stacktrace is
  defined.