Use vim_strchr(s, c) when c may be NUL (#6656)

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

src/nvim/ex_getln.c

@@ -584,7 +584,7 @@ static int command_line_execute(VimState *state, int key)
         }
         if (vim_ispathsep(ccline.cmdbuff[s->j])
 #ifdef BACKSLASH_IN_FILENAME
-            && strchr(" *?[{`$%#", ccline.cmdbuff[s->j + 1])
+            && vim_strchr(" *?[{`$%#", ccline.cmdbuff[s->j + 1])
             == NULL
 #endif
             ) {