Fix dynamic array not zeroing when growing

[Barinzaya]

When a dynamic array has unused capacity and is resized to a size greater than its capacity, the unused part of its capacity wasn't being zeroed.

Sample reproducing the issue:

package main
import "core:fmt"
main :: proc() {
	a := [dynamic]int { 1, 2, 3 }
	assert(cap(a) == 3)
	clear(&a)
	resize(&a, 4)
	fmt.println(a) // [1, 2, 3, 0] before changes, [0, 0, 0, 0] after
}