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 */

src/zmalloc.c

@@ -164,7 +164,7 @@ void *zrealloc(void *ptr, size_t size) {
     if (!newptr) zmalloc_oom_handler(size);
 
     *((size_t*)newptr) = size;
-    update_zmalloc_stat_free(oldsize);
+    update_zmalloc_stat_free(oldsize+PREFIX_SIZE);
     update_zmalloc_stat_alloc(size+PREFIX_SIZE);
     return (char*)newptr+PREFIX_SIZE;
 #endif
@@ -438,4 +438,20 @@ size_t zmalloc_get_memory_size(void) {
 #endif
 }
 
+#ifdef REDIS_TEST
+#define UNUSED(x) ((void)(x))
+int zmalloc_test(int argc, char **argv) {
+    void *ptr;
 
+    UNUSED(argc);
+    UNUSED(argv);
+    printf("Initial used memory: %zu\n", zmalloc_used_memory());
+    ptr = zmalloc(123);
+    printf("Allocated 123 bytes; used: %zu\n", zmalloc_used_memory());
+    ptr = zrealloc(ptr, 456);
+    printf("Reallocated to 456 bytes; used: %zu\n", zmalloc_used_memory());
+    zfree(ptr);
+    printf("Freed pointer; used: %zu\n", zmalloc_used_memory());
+    return 0;
+}
+#endif

src/zmalloc.h

@@ -103,4 +103,8 @@ size_t zmalloc_usable(void *ptr);
 #define zmalloc_usable(p) zmalloc_size(p)
 #endif
 
+#ifdef REDIS_TEST
+int zmalloc_test(int argc, char **argv);
+#endif
+
 #endif /* __ZMALLOC_H */