mingw: avoid a buffer overrun in `needs_hiding()`

When this function is passed a path with a trailing slash, it runs right
over the end of that path.

Let's fix this.

Co-authored-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -363,6 +363,8 @@ static inline int needs_hiding(const char *path)
 			/* ignore trailing slashes */
 			if (*path)
 				basename = path;
+			else
+				break;
 		}
 
 	if (hide_dotfiles == HIDE_DOTFILES_TRUE)