UB in derivation.c

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Hello,
I think I found undefined behaviour in the function all_spaces in the file
derivation.c
I believe there is a missing cast to unsigned char before the call to the
function c_isspace (see here:
https://git.savannah.gnu.org/cgit/bison.git/tree/src/derivation.c#n166).

I searched all usages of c_isspace in the source code of bison and this is
the only occurrence the cast to unsigned char is missing so I think that
should be fixed.

This is my first time sending a patch via email to a gnu project so I hope
this is the correct way to go about it.
I am unable to figure out how checkout the cvs repository locally, so I
git-cloned the official git repository and made a patch from there. I will
attach the patch to this email. I sincerely hope this is sufficient enough
for me to help.

Best,
Edoardo

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