mesa: fix _math_matrix_copy(), again

The matrix is 16 GLfloats in size.  Since from->inv is just a pointer (not
an array), sizeof(*from->inv) wasn't right.

src/mesa/math/m_matrix.c

@@ -1437,7 +1437,7 @@ void
 _math_matrix_copy( GLmatrix *to, const GLmatrix *from )
 {
    memcpy( to->m, from->m, sizeof(Identity) );
-   memcpy(to->inv, from->inv, sizeof(*from->inv));
+   memcpy(to->inv, from->inv, 16 * sizeof(GLfloat));
    to->flags = from->flags;
    to->type = from->type;
 }