func_strings: HASHKEY - negative array index can cause corruption

This patch makes it so only matching non-empty key names, and keys created by
the HASH function are eligible for inclusion in the comma separated string. It
also fixes a bug where it was possible to write to a negative index if the
result buffer was empty.

ASTERISK-28159
patches:
  ASTERISK-28159.diff submitted by Michael Walton (license 6502)

Change-Id: I6e57fe7307dfd856271753aed5ba64c59b511487