ofp-parse: Remove double uninit of group mod if parsing fails.

ofp-parse: Remove double uninit of group mod if parsing fails.

Double ofputil_uninit_group_mod() used to be harmless, but leads to
double free after commit e8dba7197, which will crash if any error in
group parsing happens.

Add a test to prevent this regression from happening again.

Fixes: e8dba7197 ("meta-flow: Compact struct field_array.")
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>