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

Generalize isalloc() to handle NULL pointers in such a way that the NULL
checking overhead is only paid when introspecting huge allocations (or
NULL).  This allows free() and malloc_usable_size() to no longer check
for NULL.

Submitted by Igor Bukanov and Mike Hommey.

test/null.c

@@ -0,0 +1,19 @@
+#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

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