Use constants referring to the signed integer types when setting SSIZE_T_MAX.

In practice, the sizes of the signed and unsigned integer types will
almost cetainly be the same, but this is more correct.
Pointed out in issue #638.

json_object.c

@@ -41,11 +41,11 @@
 
 #ifndef SSIZE_T_MAX
 #if SIZEOF_SSIZE_T == SIZEOF_INT
-#define SSIZE_T_MAX UINT_MAX
+#define SSIZE_T_MAX INT_MAX
 #elif SIZEOF_SSIZE_T == SIZEOF_LONG
-#define SSIZE_T_MAX ULONG_MAX
+#define SSIZE_T_MAX LONG_MAX
 #elif SIZEOF_SSIZE_T == SIZEOF_LONG_LONG
-#define SSIZE_T_MAX ULLONG_MAX
+#define SSIZE_T_MAX LLONG_MAX
 #else
 #error Unable to determine size of ssize_t
 #endif