We got some errors when some of the key values exceeded the Xapian
maximum; in particular the message-id.
So make all the key-methods check, and truncate the message-id if
necessary.
The core dump only seems to occur if mu4e-headers-include-related is
set to t.
Apparently, std::string's c_str() method is confusing to many
people, c.f.
http://stackoverflow.com/questions/22330250/how-to-return-a-stdstring-c-str
The answer seems to be that the pointer c_str() returns may not be
valid past the current statement; returning it, or even using it
subsequently can have you sending a wild pointer into e.g. g_strdup().
In short, it seems idioms like this are okay:
return g_strcmp0 (s1.c_str(), s2.c_str()) < 0;
Whereas idioms like this are not:
const char *msgid (iter->msgid().c_str());
return msgid ? g_strdup (msgid) : NULL;
At least in my environment by the time we get to g_strdup() the
pointer returned by c_str() is wild and points at garbage. Since
g_strdup() returns NULL if passed NULL, it seems collapsing it into a
single line is not only possible but necessary.
I've looked at all of the calls to c_str() in mu and it appears to
me this was the one remaining one that was bad.
