vim-patch:8.0.1512: warning for possibly using NULL pointer

Problem:    Warning for possibly using NULL pointer. (Coverity)
Solution:   Skip using the pointer if it's NULL.
e4db7aedab

src/nvim/path.c

@@ -52,7 +52,8 @@
 /// @param s2 Second file name.
 /// @param checkname When both files don't exist, only compare their names.
 /// @return Enum of type FileComparison. @see FileComparison.
-FileComparison path_full_compare(char_u *s1, char_u *s2, int checkname)
+FileComparison path_full_compare(char_u *const s1, char_u *const s2,
+                                 const bool checkname)
 {
   assert(s1 && s2);
   char_u exp1[MAXPATHL];