Fix usingJEMalloc with Clang

Fix usingJEMalloc with Clang

Summary: Clang is too clever and in some contexts optimizes away the `malloc`, but we rely on a side-effect. Declaring the variable as static forces it to call `malloc`. We could free the pointer relying on the fact that the lambda is guaranteed to be called only once, but I feel more comfortable just leaking it (LSan won't complain).

Reviewed By: philippv

Differential Revision: D2674769

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