Fix mosquitto_string_option(, MOSQ_OPT_TLS_KEYFORM, ) return value

`mosquitto_string_option(mosq, MOSQ_OPT_TLS_KEYFORM, ...)` was incorrectly
returning `MOSQ_ERR_INVAL` with valid input. This has been fixed.

Closes #1360. Thanks to Michael Dombrowski.

ChangeLog.txt

@@ -13,6 +13,9 @@ Client library:
 - Make behaviour of `mosquitto_connect_async()` consistent with
   `mosquitto_connect()` when connecting to a non-existent server.
   Closes #1345.
+- `mosquitto_string_option(mosq, MOSQ_OPT_TLS_KEYFORM, ...)` was incorrectly
+  returning `MOSQ_ERR_INVAL` with valid input. This has been fixed.
+  Closes #1360.
 
 Clients:
 - mosquitto_pub: fix error codes not being returned when mosquitto_pub exits.

lib/options.c

@@ -289,6 +289,7 @@ int mosquitto_string_option(struct mosquitto *mosq, enum mosq_opt_t option, cons
 			}else{
 				return MOSQ_ERR_INVAL;
 			}
+			return MOSQ_ERR_SUCCESS;
 #else
 			return MOSQ_ERR_NOT_SUPPORTED;
 #endif
@@ -322,8 +323,6 @@ int mosquitto_string_option(struct mosquitto *mosq, enum mosq_opt_t option, cons
 		default:
 			return MOSQ_ERR_INVAL;
 	}
-
-	return MOSQ_ERR_INVAL;
 }