Fix null pointer dereference when using "simple" EC.

This regressed in f0523e9f2046a249210c6dd86fdcbe069ea9912d.

Change-Id: I70c3fcb0d91ac00e5088b086312384756eda6140
Reviewed-on: https://boringssl-review.googlesource.com/6481
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/ec/simple.c b/crypto/ec/simple.c
index 7e611eb..f0f9d2d 100644
--- a/crypto/ec/simple.c
+++ b/crypto/ec/simple.c
@@ -83,8 +83,8 @@
                                 ec_GFp_simple_group_copy,
                                 ec_GFp_simple_group_set_curve,
                                 ec_GFp_simple_point_get_affine_coordinates,
-                                0 /* mul */,
-                                0 /* precompute_mult */,
+                                ec_wNAF_mul /* XXX: Not constant time. */,
+                                ec_wNAF_precompute_mult,
                                 ec_GFp_simple_field_mul,
                                 ec_GFp_simple_field_sqr,
                                 0 /* field_encode */,