mailmap: always store allocated strings in mailmap blob

mailmap: always store allocated strings in mailmap blob

Same as with the preceding commit, the `git_mailmap_blob` may sometimes
contain an allocated string and sometimes it may contain a string
constant. This is risky and can easily lead to bugs in case the variable
is getting re-assigned, where the code may then try to free the previous
value to avoid memory leaks.

Safeguard the code by always storing allocated strings in the variable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>