patch 9.1.1722: compiler may optimize away clearing of crypt key

Problem:  Compiler may optimize away clearing of crypt key
Solution: Use sodium_memzero() if available, else use memset() using a
          volatile function pointer (ashamedbit).

closes: #18173

Signed-off-by: ashamedbit <muralianiruddhan@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/crypt.c

@@ -780,12 +780,17 @@ crypt_decode_inplace(
     void
 crypt_free_key(char_u *key)
 {
-    char_u *p;
-
+    // Create a safe memset which cannot be optimized away by compiler
+    static void *(* volatile vim_memset_safe)(void *s, int c, size_t n) =
+	memset;
     if (key != NULL)
     {
-	for (p = key; *p != NUL; ++p)
-	    *p = 0;
+#ifdef FEAT_SODIUM
+	if (sodium_init() >= 0)
+	    sodium_memzero(key, STRLEN(key));
+	else
+#endif
+	    vim_memset_safe(key, 0, STRLEN(key));
 	vim_free(key);
     }
 }

src/version.c

@@ -724,6 +724,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1722,
 /**/
     1721,
 /**/