MFC r321587: cc_cubic: restore braces around if-condition block

r307901 was reverted in r321480, restoring an incorrect block
delimitation bug present in the original cc_cubic commit. Restore
only the bugfix (brace addition) from r307901.

[HEAD revs above; r307901 and r321480 were both merged to stable/11
in r330445.]

CID: 1090182
Reported by: bz
Sponsored by: The FreeBSD Foundation