Commit 084e1d86 added a preallocation to an array to avoid
reallocations later on, but neglected the fact that after N insertions
into the array, there’s always a NULL terminator added to the end. Fix
the preallocation to include that NULL terminator.

This doesn’t change the correctness of the code, but should eliminate
one reallocation.

Spotted by Sebastian Dröge. See
!674

.

Signed-off-by: Philip Withnall <withnall@endlessm.com>