staging: rtl8723bs: avoid null pointer dereference on pmlmepriv

There is a check to see if pmlmepriv is null before vfree'ing
pmlmepriv->free_bss_buf hence implying pmlmepriv could potenially be
null. However, a previous call to rtw_free_mlme_priv_ie_data
can also dereference pmlmepriv, so move this call so that it is only
called if pmlmepriv non-null.

Detected by CoverityScan, CID#1077739 ("Dereference before null check")

Fixes: 554c0a3a

 ("staging: Add rtl8723bs sdio wifi driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>