Otherwise, if src_bit_size > 0 and dst_bit_size == 0, we end up doing a
bad shift in `1 << (dst_bit_size - 1)`, as `dst_bit_size - 1` is a
negative value (in this case would be MAX_UINT32).

Fixes CID#1468134 "Bad bit shift operation (BAD_SHIFT)":
   "large_shift: In expression 1 << dst_bit_size - 1U, left shifting by
    more than 31 bits has undefined behavior. The shift amount,
    dst_bit_size - 1U, is 4294967295."

v2:
 - Use an assertion instead (Iago)

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <!10251>