Segfault when tagging results of to: query

[Hugh Williams]

When trying to tag all messages to a particular address using the query
`notmuch tag +College to:example at example.com` I get a segfault. The
backtrace from gdb is shown below.

```
#0  0x00007ffff78deee4 in  () at /usr/lib/libxapian.so.30
#1  0x00007ffff78e26fe in  () at /usr/lib/libxapian.so.30
#2  0x00007ffff78e90a2 in  () at /usr/lib/libxapian.so.30
#3  0x00007ffff78081f8 in Xapian::Enquire::Internal::get_mset(unsigned int, unsigned int, unsigned int, Xapian::RSet const*, Xapian::MatchDecider const*) const () at /usr/lib/libxapian.so.30
#4  0x00007ffff7808466 in Xapian::Enquire::get_mset(unsigned int, unsigned int, unsigned int, Xapian::RSet const*, Xapian::MatchDecider const*) const () at /usr/lib/libxapian.so.30
#5  0x00007ffff7fb69ca in  () at /usr/lib/libnotmuch.so.5
#6  0x000055555556bca2 in  ()
#7  0x000055555556c21a in  ()
#8  0x000055555555c12c in  ()
#9  0x00007ffff79c9153 in __libc_start_main () at /usr/lib/libc.so.6
#10 0x000055555555c2ee in  ()
```

Thanks,

Hugh