Fix pnstrdup() to not memcpy() the maximum allowed length.

The previous behaviour was dangerous if the length passed wasn't the
size of the underlying buffer, but the maximum size of the underlying
buffer.

Author: Andres Freund
Discussion: https://postgr.es/m/20161003215524.mwz5p45pcverrkyk@alap3.anarazel.de

src/backend/utils/mmgr/mcxt.c

@@ -21,6 +21,7 @@
 
 #include "postgres.h"
 
+#include "common/string.h"
 #include "miscadmin.h"
 #include "utils/memdebug.h"
 #include "utils/memutils.h"
@@ -1086,10 +1087,14 @@ pstrdup(const char *in)
 char *
 pnstrdup(const char *in, Size len)
 {
-	char	   *out = palloc(len + 1);
+	char	   *out;
 
+	len = pg_strnlen(in, len);
+
+	out = palloc(len + 1);
 	memcpy(out, in, len);
 	out[len] = '\0';
+
 	return out;
 }