patch 9.1.1402: multi-byte mappings not properly stored in session file

Problem:  multi-byte mappings not properly stored in session file
Solution: unescape the mapping before writing out the mapping, prefer
          single-byte mapping name if possible (Miguel Barro)

closes: #17355

Signed-off-by: GuyBrush <miguel.barro@live.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/misc2.c b/src/misc2.c
index 7912577..11eb51b 100644
--- a/src/misc2.c
+++ b/src/misc2.c
@@ -1213,7 +1213,7 @@
 get_special_key_name(int c, int modifiers)
 {
     static char_u string[MAX_KEY_NAME_LEN + 1];
-    int	    i, idx;
+    int	    i, idx, len;
     int	    table_idx;
 
     string[0] = '<';
@@ -1286,10 +1286,11 @@
 	// Not a special key, only modifiers, output directly
 	else
 	{
-	    if (has_mbyte && (*mb_char2len)(c) > 1)
-		idx += (*mb_char2bytes)(c, string + idx);
-	    else if (vim_isprintc(c))
+	    len = (*mb_char2len)(c);
+	    if (len == 1 && vim_isprintc(c))
 		string[idx++] = c;
+	    else if (has_mbyte && len > 1)
+		idx += (*mb_char2bytes)(c, string + idx);
 	    else
 	    {
 		char_u	*s = transchar(c);