Pacify GCC 14 on recent read_char change

Pacify GCC 14 on recent read_char change

* src/keyboard.c (read_char): Initialize c_volatile before
calling setjmp.  Although not necessary for correctness,
and not needed for gcc 14.2.1 20240801 (Red Hat 14.2.1-1)
on x86-64 when built with --enable-gcc-warnings,
some GCC 14 x86-64 configurations issue a false positive
without this change.
Problem reported by Andrea Corallo in:
https://lists.gnu.org/r/emacs-devel/2024-08/msg00620.html
and fix suggested by Pip Cet in:
https://lists.gnu.org/r/emacs-devel/2024-08/msg00627.html

(cherry picked from commit 4f1987cf77bc8e0f0180a665b3cab732021e25ab)