mesa: memset matrices at initialization to enable memcpy on it

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6946>

src/mesa/math/m_matrix.c

@@ -1519,6 +1519,7 @@ _math_matrix_loadf( GLmatrix *mat, const GLfloat *m )
 void
 _math_matrix_ctr( GLmatrix *m )
 {
+   memset(m, 0, sizeof(*m));
    memcpy( m->m, Identity, sizeof(Identity) );
    memcpy( m->inv, Identity, sizeof(Identity) );
    m->type = MATRIX_IDENTITY;