tests: Avoid hapd reference before assignment on error cases

Couple of VHT test cases missed explicit setting of hapd to None before
trying to start hostapd. Add those to avoid unexpected exceptions in
case hostapd start fails.

Signed-off-by: Jouni Malinen <j@w1.fi>

tests/hwsim/test_ap_vht.py

@@ -330,6 +330,7 @@ def test_ap_vht_40(devs, apdevs):
 def test_ap_vht_capab_not_supported(dev, apdev):
     """VHT configuration with driver not supporting all vht_capab entries"""
     try:
+        hapd = None
         params = {"ssid": "vht",
                   "country_code": "FI",
                   "hw_mode": "a",
@@ -1004,6 +1005,7 @@ def test_ap_vht_on_24ghz_2(dev, apdev):
 def test_prefer_vht40(dev, apdev):
     """Preference on VHT40 over HT40"""
     try:
+        hapd = None
         hapd2 = None
 
         params = {"ssid": "test",