[PATCH] Fix crash in c-typeck.c:comptypes() (PR3952)

[Franz Sirl]

Hi,

this little patch fixes a crash in comptypes (PR3952). AFAICS maxval can 
legally be unset here.

Bootstrapped without regressions on powerpc-linux-gnu.
OK to commit to mainline and branch?

Franz.

        PR c/3952
        * c-typeck.c (comptypes): Don't crash if maxval is unset.