Check for null pointer in CMD_START_PNO_SCAN

Catch the null pointer to prevent system server crash and enhance
logging for the future.

Bug: 268355109
Test: atest com.android.server.wifi
Test: manually verified
Change-Id: I22350983e6c24ff8399cabc56a83c8e9cddb5d86
Merged-In: I22350983e6c24ff8399cabc56a83c8e9cddb5d86
(cherry picked from commit fa3c2412a50e37740691bb1dbe495253c0b5872d)
1 file changed