Fixed ssi and perl interaction.

Embedded perl module assumes there is a space for terminating NUL character,
make sure to provide it in all situations by allocating one extra byte for
value buffer.  Default ssi_value_length is reduced accordingly to
preserve 256 byte allocations.

While here, fixed another one byte value buffer overrun possible in
ssi_quoted_symbol_state.

Reported by Matthew Daley.