runtime: zero freed memory

This helps to find bugs in the GC. It does have a performance impact so
it's only enabled when asserts are enabled.

src/runtime/gc_blocks.go

@@ -168,6 +168,9 @@ func (b gcBlock) markFree() {
 	if gcAsserts && b.state() != blockStateFree {
 		runtimePanic("gc: markFree() was not successful")
 	}
+	if gcAsserts {
+		*(*[wordsPerBlock]uintptr)(unsafe.Pointer(b.address())) = [wordsPerBlock]uintptr{}
+	}
 }
 
 // unmark changes the state of the block from mark to head. It must be marked