Remove alloca from loop and use a single fixed size array declaration

[madmiraal]

As identified by lgtm, #38645 introduced alloca in a loop. Unlike memory allocated to a variable, which is reused on every iteration, memory allocated by alloca is not freed until the calling function returns. Not only does this run the risk of causing a stack overflow, but it neither needs alloca nor be inside the loop. First, the memory can be allocated using a standard fixed size array, and second, the array can be reused.

This PR changes the declaration of argptrs to a fixed size array of const Variant pointers and moves the declaration outside the loop.

[akien-mga]

Thanks!