postgres: Use pg_{add,mul}_size_overflow()

The backend implementations of add_size() and mul_size() can now make
use of the APIs provided in common/int.h.

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/backend/storage/ipc/shmem.c

@@ -65,6 +65,7 @@
 
 #include "postgres.h"
 
+#include "common/int.h"
 #include "fmgr.h"
 #include "funcapi.h"
 #include "miscadmin.h"
@@ -495,9 +496,7 @@ add_size(Size s1, Size s2)
 {
 	Size		result;
 
-	result = s1 + s2;
-	/* We are assuming Size is an unsigned type here... */
-	if (result < s1 || result < s2)
+	if (pg_add_size_overflow(s1, s2, &result))
 		ereport(ERROR,
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("requested shared memory size overflows size_t")));
@@ -512,11 +511,7 @@ mul_size(Size s1, Size s2)
 {
 	Size		result;
 
-	if (s1 == 0 || s2 == 0)
-		return 0;
-	result = s1 * s2;
-	/* We are assuming Size is an unsigned type here... */
-	if (result / s2 != s1)
+	if (pg_mul_size_overflow(s1, s2, &result))
 		ereport(ERROR,
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("requested shared memory size overflows size_t")));