siglent-sds: Fix multiple scan-build warnings.

src/hardware/siglent-sds/api.c:596:3: warning: Argument to g_free() is the address of a global variable, which is not memory allocated by malloc()
                  g_free(cmd);
                  ^~~~~~~~~~~
  src/hardware/siglent-sds/api.c:641:3: warning: Argument to g_free() is the address of a global variable, which is not memory allocated by malloc()
                  g_free(cmd);
                  ^~~~~~~~~~~

src/hardware/siglent-sds/api.c

@@ -531,7 +531,7 @@ static int config_set(uint32_t key, GVariant *data,
 	int ret, idx;
 	const char *tmp_str;
 	char buffer[16];
-	char *cmd = "";
+	char *cmd = NULL;
 	char cmd4[4];
 
 	devc = sdi->priv;