[scudo] releaseToOSMaybe can fail if it can't allocate PageMap

PageMap is allocated with MAP_ALLOWNOMEM if there's no static buffer
left. So it can be failed and return nullptr without any assertion
triggered. Instead of crashing in the releaseToOSMaybe in the middle,
just return and let the program handles the page failure.

Reviewed By: cferris

Differential Revision: https://reviews.llvm.org/D151379