rpc: avoid undefined behaviour when parsing bindings

rpc: avoid undefined behaviour when parsing bindings

If the binding string ends with "[", we were setting options to an
empty string, then asking for 'options[strlen(options)-1]', which
UBSan dosn't like because the offset evaluates to (size_t)0xFFFFF...
causing pointer overflow.

I believe this is actually well defined in practice, but we don't want
to be in the habit of leaving sanitiser warnings in code parsing
untrusted strings.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>