patterndb: fixed a failed assertion when using builtin values in a pattern

An assertion was failed when using a db-parser() pattern that contains
a builtin name-value pair, like:

replace HOST with @ANYSTRING:HOST@

This is fixed by this patch.

Reported-By: Evan Rempel <erempel@uvic.ca>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>