As part of the refactoring in #5119, some vim_strchr() were changed to
strchr(). However, vim_strchr() behaves differently than strchr() when
c is NUL, returning NULL instead of a pointer to the NUL.
Revert the strchr() calls where it isn't known whether c is NUL, since
this causes a semantic change the surrounding code doesn't expect. In
the case of #6650, this led to a heap overrun.
Closes#6650