Switch sresize to using TYPECHECK.

Now that that facility is centralised in defs.h, there's no reason to
have a special ad-hoc version in sresize and separately comment it.

puttymem.h

@@ -22,23 +22,16 @@ void safefree(void *);
 
 /*
  * Direct use of smalloc within the code should be avoided where
- * possible, in favour of these type-casting macros which ensure
- * you don't mistakenly allocate enough space for one sort of
- * structure and assign it to a different sort of pointer.
- *
- * The nasty trick in sresize with sizeof arranges for the compiler,
- * in passing, to type-check the expression ((type *)0 == (ptr)), i.e.
- * to type-check that the input pointer is a pointer to the correct
- * type. The construction sizeof(stuff) ? (b) : (b) looks like a
- * violation of the first principle of safe macros, but in fact it's
- * OK - although it _expands_ the macro parameter more than once, it
- * only _evaluates_ it once, so it's still side-effect safe.
+ * possible, in favour of these type-casting macros which ensure you
+ * don't mistakenly allocate enough space for one sort of structure
+ * and assign it to a different sort of pointer. sresize also uses
+ * TYPECHECK to verify that the _input_ pointer is a pointer to the
+ * correct type.
  */
 #define snew(type) ((type *)snmalloc(1, sizeof(type)))
 #define snewn(n, type) ((type *)snmalloc((n), sizeof(type)))
-#define sresize(ptr, n, type)                                           \
-    ((type *)snrealloc(sizeof((type *)0 == (ptr)) ? (ptr) : (ptr),      \
-                       (n), sizeof(type)))
+#define sresize(ptr, n, type) TYPECHECK((type *)0 == (ptr), \
+    ((type *)snrealloc((ptr), (n), sizeof(type))))
 
 /*
  * For cases where you want to allocate a struct plus a subsidiary