Fix crash when converting BMesh to Mesh with shape keys

The `BM_mesh_bm_to_me()` function copies shape keys from the BMesh to
the Mesh. However, it tries to copy the same number of shape keys as are
defined on the target mesh. Since the target mesh does not necessarily
have the same number of shape keys as the BMesh, this would crash if the
target Mesh has more.

Found while performing some tests for {D7785}.

Differential Revision: https://developer.blender.org/D7818

Reviewed by: brecht

source/blender/bmesh/intern/bmesh_mesh_conv.c

@@ -893,6 +893,10 @@ void BM_mesh_bm_to_me(Main *bmain, BMesh *bm, Mesh *me, const struct BMeshToMesh
 
       j = bm_to_mesh_shape_layer_index_from_kb(bm, currkey);
       cd_shape_offset = CustomData_get_n_offset(&bm->vdata, CD_SHAPEKEY, j);
+      if (cd_shape_offset < 0) {
+        /* The target Mesh has more shapekeys than the BMesh. */
+        continue;
+      }
 
       fp = newkey = MEM_callocN(me->key->elemsize * bm->totvert, "currkey->data");
       oldkey = currkey->data;