[Bug c++/95700] read-md.c: "missing sentinel in function call" when building gcc with musl

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95700

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
To be really safe during stage 1, GCC should not use NULL as a pointer sentinel
in C++ code anyway.

The bootstrap compiler could define it to 0 or 0u, neither of which is
guaranteed to be OK to pass as a varargs sentinel where a null pointer is
expected. Any of (void*)0 or (void*)NULL or nullptr would be safe.