Port the fix from #377 to emmmalloc.

[sunfishcode]

Avoid using sbrk(0) in emmalloc too.

[jedisct1]

Unfortunately this is causing applications to trap with an out of bounds access.

[sunfishcode]

Ah, that's unfortunate. Any help anyone might be able to provide in debugging this would be appreciated!

[sbc100]

Unfortunately this is causing applications to trap with an out of bounds access.

Is that with or without the __heap_base symbol? i.e. which llvm version are you using?

[jedisct1]

This is with LLVM 15.0.6.

[jedisct1]

__heap_base is defined but not __heap_end.

[cuviper]

(irrelevant nit: the commit message and title of this PR have too many 'm's)