CLEANUP: map: always test pat->ref in sample_conv_map_key()

CLEANUP: map: always test pat->ref in sample_conv_map_key()

sample_conf_map_key() calls pattern_exec_match() which may return a
static pattern with ref=NULL when passed with fill=1 (which is the
case) and pat->match == NULL (which doesn't seem to be the case). It
doesn't seem it could happen with standard maps, as only "-m found"
drops has a NULL ->match function and there's no keyword associated
with it) but maybe this could happen with maps implemented in Lua,
though this remains unlikely.

Anyway better clarify the situation by always checking that the ref
is non-null before dereferencing it, it will at least avoid warnings
from code coverage tools.