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);
                  ^~~~~~~~~~~