Fix crash when serial data is sent on boot
Fix crash when serial data is sent on boot

Serial data sent from the target before enumeration causes DAPLink
to crash. This is because ptr_data_to_send is uninitialized. This
patch fixes the crash by initializing this and other CDC variables
in USBD_CDC_ACM_Initialize.

This problem was introduced in the commit:
fb74824abd871ebb0108409f10eb087b9e3e3e88 -
Move serial processing to main thread

Special thanks to @devanlai for discovering and reporting this.

sg-/DAPLink,google/DAPLink-port,google/DAPLink-port,sg-/DAPLink,sg-/DAPLink,google/DAPLink-port,google/DAPLink-port