don't try and free what must be a null vector when reserving 0 elements

don't try and free what must be a null vector when reserving 0 elements
in va_heap::reserve

2013-11-05  Trevor Saunders  <tsaunders@mozilla.com>

	* vec.c (vec_prefix::calculate_allocation): Don't try to handle the
	case of no prefix and reserving zero slots, because when that's the
	case we'll never get here.
	* vec.h (va_heap::reserve): Don't try and handle
	vec_prefix::calculate_allocation returning zero because that should
	never happen.

From-SVN: r204392