patch 9.1.0644: Unnecessary STRLEN() when applying mapping

Problem:  Unnecessary STRLEN() when applying mapping.
          (after v9.1.0642)
Solution: Use m_keylen and vim_strnsave().
          (zeertzjq)

closes: #15394

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    644,
 /**/
     643,
 /**/