Add pg_add_size_overflow() and friends

Commit 600086f47 added (several bespoke copies of) size_t addition with
overflow checks to libpq. Move this to common/int.h, along with
its subtraction and multiplication counterparts.

pg_neg_size_overflow() is intentionally omitted; I'm not sure we should
add SSIZE_MAX to win32_port.h for the sake of a function with no
callers.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAOYmi%2B%3D%2BpqUd2MUitvgW1pAJuXgG_TKCVc3_Ek7pe8z9nkf%2BAg%40mail.gmail.com

src/include/common/int.h

@@ -601,6 +601,73 @@ pg_neg_u64_overflow(uint64 a, int64 *result)
 #endif
 }
 
+/*
+ * size_t
+ */
+static inline bool
+pg_add_size_overflow(size_t a, size_t b, size_t *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_add_overflow(a, b, result);
+#else
+	size_t		res = a + b;
+
+	if (res < a)
+	{
+		*result = 0x5EED;		/* to avoid spurious warnings */
+		return true;
+	}
+	*result = res;
+	return false;
+#endif
+}
+
+static inline bool
+pg_sub_size_overflow(size_t a, size_t b, size_t *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_sub_overflow(a, b, result);
+#else
+	if (b > a)
+	{
+		*result = 0x5EED;		/* to avoid spurious warnings */
+		return true;
+	}
+	*result = a - b;
+	return false;
+#endif
+}
+
+static inline bool
+pg_mul_size_overflow(size_t a, size_t b, size_t *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_mul_overflow(a, b, result);
+#else
+	size_t		res = a * b;
+
+	if (a != 0 && b != res / a)
+	{
+		*result = 0x5EED;		/* to avoid spurious warnings */
+		return true;
+	}
+	*result = res;
+	return false;
+#endif
+}
+
+/*
+ * pg_neg_size_overflow is currently omitted, to avoid having to reason about
+ * the portability of SSIZE_MIN/_MAX before a use case exists.
+ */
+/*
+ * static inline bool
+ * pg_neg_size_overflow(size_t a, ssize_t *result)
+ * {
+ *     ...
+ * }
+ */
+
 /*------------------------------------------------------------------------
  *
  * Comparison routines for integer types.