write_cxxrtl: avoid undefined behavior on out-of-bounds memory access.

write_cxxrtl: avoid undefined behavior on out-of-bounds memory access.

After this commit, if NDEBUG is not defined, out-of-bounds accesses
cause assertion failures for reads and writes. If NDEBUG is defined,
out-of-bounds reads return zeroes, and out-of-bounds writes are
ignored.

This commit also adds support for memories that start with a non-zero
index (`Memory::start_offset` in RTLIL).