Verify that the selected BSS has a better signal level before roaming

This prevents situations like the following where we roam to a
lesser quality BSS just because the signal level delta is over our
threshold.

wlan0: Considering within-ESS reassociation
wlan0: Current BSS: 00:24:6c:74:0a:40 level=-51
wlan0: Selected BSS: 00:24:6c:74:0a:e0 level=-64
wlan0: Request association: reassociate: 0  selected: 00:24:6c:74:0a:e0
    bssid: 00:24:6c:74:0a:40  pending: 00:00:00:00:00:00  wpa_state: COMPLETED

Signed-hostap: Robert Shade <robert.shade@gmail.com>

wpa_supplicant/events.c

@@ -1025,6 +1025,12 @@ static int wpa_supplicant_need_to_roam(struct wpa_supplicant *wpa_s,
 		return 1;
 	}
 
+	if (current_bss->level < 0 && current_bss->level > selected->level) {
+		wpa_dbg(wpa_s, MSG_DEBUG, "Skip roam - Current BSS has better "
+			"signal level");
+		return 0;
+	}
+
 	min_diff = 2;
 	if (current_bss->level < 0) {
 		if (current_bss->level < -85)