Fix wrong size in K_BHeapPush Z_Realloc

Fixes K_BHeapPush's Z_Realloc not having a sizeof(bheapitem_t) when it did a realloc. This meant that it would realloc 16, 32... bytes instead of 16/32 * sizeof(bheapitem_t) bytes, causing access violations.

This could cause functions that used K_PathfindAStar to crash, such as the SPB's pathfinding, on some certain cases.

Before:

After:

Trivial fix, no build.