Added simple test for ca_hashmap_*

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
hashmap.c: Correctly hash zero-length strings

Previously, the hash function would read past the end of the string.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
tests/hashmap-00.c: Test for non-existence of non-inserted items

This improves the coverage of the test.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>