[PATCH next] don't let mailmap provoke use of freed memory

On an x86_64 system (F13-based), I ran these commands in an empty directory:

    git init
    printf '%s\n' \
      '[off-list ref] [off-list ref]' \
      'John [off-list ref]' > .mailmap
    git shortlog < /dev/null

Here's the result:

    (reading log message from standard input)
    *** glibc detected *** git: free(): invalid pointer: 0x0000000000f53730 ***
    ======= Backtrace: =========
    /lib64/libc.so.6[0x31ba875676]
    git[0x48c2a5]
    git[0x4b9858]
    ...
    zsh: abort (core dumped)  git shortlog

What happened?

Some .mailmap entry is of the <email1> <email2> form,
while a subsequent one looks like "User Name <Email2>,
and the two email addresses on the right are not identical
but are "equal" when using a case-insensitive comparator.

Then, when add_mapping is processing the latter line, new_email is NULL
and we free me->email, yet do not replace it with a new strdup'd string.
Thus, when later we attempt to use the buffer behind that ->email pointer,
we reference freed memory.

The solution is to free ->email and ->name only if we're about to replace them.

Signed-off-by: Jim Meyering <redacted>
---
 mailmap.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/mailmap.c b/mailmap.c
index f80b701..02fcfde 100644
--- a/mailmap.c
+++ b/mailmap.c

@@ -79,12 +79,14 @@ static void add_mapping(struct string_list *map,
 	if (old_name == NULL) {
 		debug_mm("mailmap: adding (simple) entry for %s at index %d\n", old_email, index);
 		/* Replace current name and new email for simple entry */
-		free(me->name);
-		free(me->email);
-		if (new_name)
+		if (new_name) {
+			free(me->name);
 			me->name = xstrdup(new_name);
-		if (new_email)
+		}
+		if (new_email) {
+			free(me->email);
 			me->email = xstrdup(new_email);
+		}
 	} else {
 		struct mailmap_info *mi = xmalloc(sizeof(struct mailmap_info));
 		debug_mm("mailmap: adding (complex) entry for %s at index %d\n", old_email, index);
--

1.7.3.1.104.gc752e