rcfile: reject key names that are wrong or too long

Meta keys should have a dash as second character, and apart from ^Space
and M-Space key names should be at most two or three characters long.

This fixes https://savannah.gnu.org/bugs/?44688

.
Reviewed-by: Benno Schulenberg <bensberg@justemail.net>
Signed-off-by: Rishabh Dave <rishabhddave@gmail.com>

src/rcfile.c

@@ -399,7 +399,11 @@ void parse_binding(char *ptr, bool dobind)
     else if (keycopy[0] != '^' && keycopy[0] != 'M' && keycopy[0] != 'F') {
 	rcfile_error(N_("Key name must begin with \"^\", \"M\", or \"F\""));
 	goto free_copy;
-    } else if (keycopy[0] == '^' && (keycopy[1] < 64 || keycopy[1] > 127)) {
+    } else if ((keycopy[0] == 'M' && keycopy[1] != '-') ||
+		(keycopy[0] == '^' && ((keycopy[1] < 64 || keycopy[1] > 127) ||
+		(strlen(keycopy) > 2 && strcmp(keycopy, "^Space") != 0))) ||
+		(strlen(keycopy) > 3 && strcmp(keycopy, "^Space") != 0 &&
+		strcmp(keycopy, "M-Space") != 0)) {
 	rcfile_error(N_("Key name %s is invalid"), keycopy);
 	goto free_copy;
     }