Fix incorrect return value in JSON equality function for scalars

equalsJsonbScalarValue() uses a boolean as return type, however for one
code path -1 gets returned, which is confusing.  The origin of the
confusion is visibly that this code got copy-pasted from
compareJsonbScalarValue() since it has been introduced in d1d50bff.

No backpatch, as this is only cosmetic.

Author: Rikard Falkeborn
Discussion: https://postgr.es/m/CADRDgG7mJnek6HNW13f+LF6V=6gag9PM+P7H5dnyWZAv49aBGg@mail.gmail.com

src/backend/utils/adt/jsonb_util.c

@@ -1318,7 +1318,7 @@ equalsJsonbScalarValue(JsonbValue *aScalar, JsonbValue *bScalar)
 		}
 	}
 	elog(ERROR, "jsonb scalar type mismatch");
-	return -1;
+	return false;
 }
 
 /*