If MBS conversion fails while parsing a bracketed expression, we'd end
up writing '\0' to `buf[-1]`.  Check the result of the conversion.

While here, rewrite `tre_parse_bracket_items()` to return immediately
on error, instead of lazily checking the status later.

This fixes laurikari#51.

Since we now return immediately on error, we don't need an if-else chain.
This allows us to unindent a large swath of code.

I meant to include this in a0cafd4 but forgot.