Fix uninitialized frame number

[fightingdreamer]

Frame number was not initialised in AsyncWebSocketClient.
This was resulting in bogus frame number during first message.

example: one frame 2000 bytes

# first message
     ws: frame=1162280960 length=2000 index=0 chunk=528 final=1 opcode=2
     ws: frame=1162280960 length=2000 index=528 chunk=536 final=1 opcode=2
     ws: frame=1162280960 length=2000 index=1064 chunk=536 final=1 opcode=2
     ws: frame=1162280960 length=2000 index=1600 chunk=400 final=1 opcode=2

# first message (fixed)
     ws: frame=0 length=2000 index=0 chunk=528 final=1 opcode=2
     ws: frame=0 length=2000 index=528 chunk=536 final=1 opcode=2
     ws: frame=0 length=2000 index=1064 chunk=536 final=1 opcode=2
     ws: frame=0 length=2000 index=1600 chunk=400 final=1 opcode=2

[fightingdreamer]

Related to #668.