libphobos: Don't call free on the TLS array in the emutls destroy function.

Fixes a segfault seen on Darwin when a GC scan is ran after a thread has
been destroyed.  As the global emutlsArrays hash still has a reference
to the array itself, and tries to iterate all elements.

Setting the length to zero frees all allocated elements in the array,
and ensures that it is skipped when the _d_emutls_scan is called.

libphobos/ChangeLog:

	* libdruntime/gcc/emutls.d (emutlsDestroyThread): Clear the per-thread
	TLS array, don't call free().