In release 1.0 of the WebAssembly Specification, it says that all the
bytes in WebAssembly memory need to be initialized to 0 on creation of
the memory, and when memory is grown, the new bytes also need to be
initialized to 0.
It seems this zeroing behaviour is indeed required for the sandbox to
operate correctly. Not zeroing leads to undefined behaviour. This
manifested as a crash that occurred when restarting the libretro core,
but for some reason, only on Emscripten. Not sure why this didn't happen
on other platforms. Even sanitizers weren't able to detect the bug!
(cherry picked from commit edf061e323b8f0ab0c6a72c76ae7ccc07a1649c0)
According to AddressSanitizer, when `sandbox_malloc` causes the
WebAssembly memory to grow in size, every single coroutine on the
sandbox stack gets corrupted. So if `sandbox_malloc` is going to cause
the memory to grow in size, we need to yield so that there are no
coroutines on the sandbox stack while the reallocation occurs.
