make config --add behave correctly for empty and NULL values

Currently if we have a config file like,
[foo]
        baz
        bar =

and we try something like, "git config --add foo.baz roll", Git will
segfault. Moreover, for "git config --add foo.bar roll", it will
overwrite the original value instead of appending after the existing
empty value.

The problem lies with the regexp used for simulating --add in
`git_config_set_multivar_in_file()`, "^$", which in ideal case should
not match with any string but is true for empty strings. Instead use a
regexp like "a^" which can not be true for any string, empty or not.

For removing the segfault add a check for NULL values in `matches()` in
config.c.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/config.c

@@ -599,7 +599,7 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 		check_argc(argc, 2, 2);
 		value = normalize_value(argv[0], argv[1]);
 		return git_config_set_multivar_in_file(given_config_source.file,
-						       argv[0], value, "^$", 0);
+						       argv[0], value, "a^", 0);
 	}
 	else if (actions == ACTION_REPLACE_ALL) {
 		check_write();