Don’t ignore -Wclobbered in image.c

Don’t ignore -Wclobbered in image.c

This fix is also prompted by Emacs bug#71744.
* src/image.c: Do not ignore -Wclobbered.
(png_load_body): Fix violations of the C standard, where setjmp
clobbered c.  Move mask_img decl to pacify GCC.
(jpeg_load_body): Don’t make fp volatile; solve that problem in a
better way, via a new fp_volatile local.  Fix violations of the C
standard, where setjmp clobbered mgr, img, and ximg.  If __GNUC__
&& !__clang__, add useless assignments to pacify GCC.

(cherry picked from commit 3b24ac538858d994a74826361a1af3f802dd065a)