_gcry_burn_stack: use memset for clearing memory

* src/misc.c (__gcry_burn_stack) [HAVE_VLA]: Use 'memset' for clearing
stack.

Patch switches stacking burning to use faster memset instead of
wipememory. Memset is accessed through volatile function pointer,
so that compiler will not optimize away the call.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>