Don't crash on very long inputs

Because our style engine uses 16 bit indexes, strings can only be
styled if they're shorter than 2^16 characters. We do check for this
in the style engine and refuse to style strings that are longer.

However, just returning an default-constructed StyledString() is wrong,
because other places rely on there being at least one format and the
plaintext be initialized. So the proper way of handling this is just
using the baseFormat and the full string as plaintext instead of an
empty StyledString.

Fixes #1257.