MemArena: Fix a memory leak caused by pointer confusion

This code was ported from out_ptr, which was a double-pointer, and
wanted to double-check that the proper arena was actually allocated.

When I ported it to store the pointer directly in the view regardless
of whether out_ptr was non-NULL, I got confused here and instead
caused the code to only free the arena if the first byte was non-zero.

Spotted by @comex on IRC. Nice catch.