ensure we "clear the cofactor" when mapping field to curve point

Author: ralexstokes

eth/precompiles/bls.py

@@ -208,7 +208,8 @@ def _parse_fp2_element(data: bytes) -> bls12_381.FQ2:
 
 def _map_fp2_to_g2(field_element: bls12_381.FQ2) -> G2Point:
     point = bls.hash_to_curve.map_to_curve_G2(field_element)
-    return bls12_381.normalize(point)
+    group_element = bls.hash_to_curve.clear_cofactor_G2(point)
+    return bls12_381.normalize(group_element)
 
 
 def map_fp2_to_g2(computation: BaseComputation,