bug#58096: support malloc NULL return

[Aleksandar Kostadinov]

Hello,

grep fails when using a certain malloc implementation (memkind) because of
its reliance on malloc returning 0 when zero size is passed to it. Memkind
returns NULL instead which according to authors [1] is one of the
acceptable values in malloc specification.

Can grep be updated to also support a NULL return value?

Thank you!

[1] https://github.com/memkind/memkind/issues/853#issuecomment-1241678167