Use Size rather than int for storing strnlen's return

... in fe_memutils.c's pnstrdup().

strnlen() does return size_t, so it seems better to use the Size input
parameter to store the return value rather than demote it to int and
risk larger values being truncated.

Pushing only to master as this doesn't seem like an actual problem.
Fixing will hopefully prevent future LLM-triggered reports about this.

Author: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/0E717E09-809F-4BC4-B3B1-36BE729DBFAE@gmail.com

src/common/fe_memutils.c

@@ -151,7 +151,6 @@ char *
 pnstrdup(const char *in, Size size)
 {
 	char	   *tmp;
-	int			len;
 
 	if (!in)
 	{
@@ -160,16 +159,16 @@ pnstrdup(const char *in, Size size)
 		exit(EXIT_FAILURE);
 	}
 
-	len = strnlen(in, size);
-	tmp = malloc(len + 1);
+	size = strnlen(in, size);
+	tmp = malloc(size + 1);
 	if (tmp == NULL)
 	{
 		fprintf(stderr, _("out of memory\n"));
 		exit(EXIT_FAILURE);
 	}
 
-	memcpy(tmp, in, len);
-	tmp[len] = '\0';
+	memcpy(tmp, in, size);
+	tmp[size] = '\0';
 
 	return tmp;
 }