Ensure a resized empty buffer is null-terminated

Ensure a resized empty buffer is null-terminated

The new buffer code is using the pool, which ensures its buffers are
null-terminated.

However, if a "new" buffer from another part of the code were passed
to one of the temporary interfaces that resizes the buffer, it's
possible a non-terminated string might end up being passed through.

It's reasonable to expect mutt_b2s() for a "new" buffer should be the
same as after it is resized larger.  So ensure the resulting buf->data
is properly terminated to avoid surprises.