config: let `config_store_data_clear()` handle `value_regex`

config: let `config_store_data_clear()` handle `value_regex`

Instead of duplicating the logic for clearing up `value_regex`, let
`config_store_data_clear()` handle that.

When `regcomp()` fails, the current code does not call `regfree()`. Make
sure we do the same by immediately invalidating `value_regex`. Some
implementations are able to handle such an extra `regfree()`-call [1],
but from the example in [2], we should not do so. (The language itself
in [2] is not super-clear on this.)

[1] https://www.redhat.com/archives/libvir-list/2013-September/msg00262.html

[2] http://pubs.opengroup.org/onlinepubs/9699919799/functions/regcomp.html

Researched-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>