patch 8.2.4504: when there is a partially matching map full map may not work

Problem:    When there is a partially matching map and modifyOtherKeys is
            active a full map may not work.
Solution:   Only simplify modifiers when there is no matching mapping.
            (closes #8792)
diff --git a/src/getchar.c b/src/getchar.c
index 2fbbd4f..03d6d4d 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -2598,7 +2598,7 @@
 	}
 
 	// If no partly match found, use the longest full match.
-	if (keylen != KEYLEN_PART_MAP)
+	if (keylen != KEYLEN_PART_MAP && mp_match != NULL)
 	{
 	    mp = mp_match;
 	    keylen = mp_match_len;
@@ -2643,7 +2643,7 @@
 	    max_mlen = mlen + 1;
     }
 
-    if ((mp == NULL || max_mlen >= mp_match_len) && keylen != KEYLEN_PART_MAP)
+    if ((mp == NULL || max_mlen > mp_match_len) && keylen != KEYLEN_PART_MAP)
     {
 	int	save_keylen = keylen;