Harden usingJEMalloc()

Harden usingJEMalloc()

Summary:
In a shared library, we're using whichever memory allocator the main program
uses, but we may still believe we're using jemalloc if we load libjemalloc.so
(because rallocm is defined). This means we'll pass to rallocm pointers that
were not allocated by jemalloc, which is bad.

Test Plan: tests added

Reviewed By: ngbronson@fb.com

FB internal diff: D1228483