Revert "Avoid NULL check in free() and malloc_usable_size()."

This reverts commit 96d4120ac0.

ivsalloc() depends on chunks_rtree being initialized.  This can be
worked around via a NULL pointer check.  However,
thread_allocated_tsd_get() also depends on initialization having
occurred, and there is no way to guard its call in free() that is
cheaper than checking whether ptr is NULL.

test/null.c

@@ -1,19 +0,0 @@
-#include <stdio.h>
-#include <assert.h>
-
-#define	JEMALLOC_MANGLE
-#include "jemalloc_test.h"
-
-int
-main(void)
-{
-
-	fprintf(stderr, "Test begin\n");
-
-	free(malloc(1));
-	free(NULL);
-	assert(malloc_usable_size(NULL) == 0);
-
-	fprintf(stderr, "Test end\n");
-	return (0);
-}

test/null.exp

@@ -1,2 +0,0 @@
-Test begin
-Test end