Set SRTP context to NULL after deallocation

[sauwming]

Currently pjmedia_transport_srtp_stop() will deallocate the SRTP contexts but doesn't reset them to NULL. This can potentially cause crash in pjmedia_transport_get_info() which will call srtp_get_stream_roc() to the contexts.

 #00  pc 0000000000219c5c 
/data/app/~~3jcI6mCrsbub2EXvTHLuXQ==/split_config.arm64_v8a.apk!lib/arm64-v8a/libwwpjsua2.so 
(offset 0x331000) (srtp_get_stream_roc+28)
   #00  pc 00000000001e00a4 
/data/app/~~3jcI6mCrsbub2EXvTHLuXQ==/split_config.arm64_v8a.apk!lib/arm64-v8a/libwwpjsua2.so 
(offset 0x331000)
   #00  pc 0000000000182220 
/data/app/~~3jcI6mCrsbub2EXvTHLuXQ==/split_config.arm64_v8a.apk!lib/arm64-v8a/libwwpjsua2.so 
(offset 0x331000) (pjsua_call_dump+908)
   #00  pc 000000000017b8cc 
/data/app/~~3jcI6mCrsbub2EXvTHLuXQ==/split_config.arm64_v8a.apk!lib/arm64-v8a/libwwpjsua2.so 
(offset 0x331000) (pjsua_media_channel_deinit+280)