Fix #1153 and #1198

In P_LoadExtraVertices, the old code reallocated vertexes and for each line did this:

line->v1 += (size_t)(new_pointer_to_vertexes - old_pointer_to_vertexes);
line->v2 += (size_t)(new_pointer_to_vertexes - old_pointer_to_vertexes);

Now instead, it calculates the index of the vertex and reassigns them to the reallocated vertexes memory.