func_db: Add validity check for key names when writing.

Adds a simple sanity check for key names when users are
writing data to AstDB. This captures four cases indicating
malformed keynames that generally result in bad data going
into the DB that the user didn't intend: an empty key name,
a key name beginning or ending with a slash, and a key name
containing two slashes in a row. Generally, this is the
result of a variable being used in the key name being empty.

If a malformed key name is detected, a warning is emitted
to indicate the bug in the dialplan.

ASTERISK-29925 #close

Change-Id: Ifc08a9fe532a519b1b80caca1aafed7611d573bf