Copy dba_*() keys before converting to string.

A nice Sunday afternoon project for somebody would be to refactor the dba
functions to use zend_parse_parameters() reliably and try to untangle some of
the macros in dba.c. Sadly, it is not a nice Sunday afternoon here.

Fixes bug #65708 (dba functions cast $key param to string in-place, bypassing
copy on write).