strbuf_test: Added a couple of tests operating on a zero-size buffer.
strbuf_test: Added a couple of tests operating on a zero-size buffer.

This is mostly about checking that this does not crash.

tokkee/sysdb,sysdb/sysdb,sysdb/sysdb,tokkee/sysdb,sysdb/sysdb,tokkee/sysdb