FIX: crash on Bionic libc if CloseProxmark is called twice. (#672)

In Android O and later, if an invalid pthread_t is passed to pthread_join,
it calls fatal().

https://github.com/aosp-mirror/platform_bionic/blob/ed16b344e75f422fb36fbfd91fb30de339475880/libc/bionic/pthread_internal.cpp#L116-L128

This patch addresses it by:

1. Always memset(0) on USB_communications_thread at the end of
   CloseProxmark.

2. On Bionic, only call pthread_join on USB_communications_thread if it is
   not equal to 0.