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>
diff --git a/src/version.c b/src/version.c
index bfcb7bf..a266286 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    644,
+/**/
     643,
 /**/
     642,