Fix incorrect memory usage accounting in zrealloc

When HAVE_MALLOC_SIZE is false, each call to zrealloc causes used_memory
to increase by PREFIX_SIZE more than it should, due to mis-matched
accounting between the original zmalloc (which includes PREFIX size in
its increment) and zrealloc (which misses it from its decrement).

I've also supplied a command-line test to easily demonstrate the
problem. It's not wired into the test framework, because I don't know
TCL so I'm not sure how to automate it.

src/server.c

@@ -4015,6 +4015,8 @@ int main(int argc, char **argv) {
             return endianconvTest(argc, argv);
         } else if (!strcasecmp(argv[2], "crc64")) {
             return crc64Test(argc, argv);
+        } else if (!strcasecmp(argv[2], "zmalloc")) {
+            return zmalloc_test(argc, argv);
         }
 
         return -1; /* test not found */