patch 8.0.0212: buffer for key name may be too small

Problem:    The buffer used to store a key name theoreticaly could be too
            small. (Coverity)
Solution:   Count all possible modifier characters.  Add a check for the
            length just in case.

src/misc2.c

@@ -2162,6 +2162,7 @@ static struct modmasktable
     /* 'A' must be the last one */
     {MOD_MASK_ALT,		MOD_MASK_ALT,		(char_u)'A'},
     {0, 0, NUL}
+    /* NOTE: when adding an entry, update MAX_KEY_NAME_LEN! */
 };
 
 /*
@@ -2431,6 +2432,7 @@ static struct key_name_entry
     {K_PLUG,		(char_u *)"Plug"},
     {K_CURSORHOLD,	(char_u *)"CursorHold"},
     {0,			NULL}
+    /* NOTE: When adding a long name update MAX_KEY_NAME_LEN. */
 };
 
 #define KEY_NAMES_TABLE_LEN (sizeof(key_names_table) / sizeof(struct key_name_entry))
@@ -2659,8 +2661,13 @@ get_special_key_name(int c, int modifiers)
     }
     else		/* use name of special key */
     {
-	STRCPY(string + idx, key_names_table[table_idx].name);
-	idx = (int)STRLEN(string);
+	size_t len = STRLEN(key_names_table[table_idx].name);
+
+	if (len + idx + 2 <= MAX_KEY_NAME_LEN)
+	{
+	    STRCPY(string + idx, key_names_table[table_idx].name);
+	    idx += (int)len;
+	}
     }
     string[idx++] = '>';
     string[idx] = NUL;