force wait state after lib_cancel_transfer to avoid crash

[jvde-github]

Good day!

I was struggling with an occasional crash of the rtl-sdr tools on windows when closing the device after pressing CTRL-C. Both with the versions as provided with VCPKG and when directly compiling the latest version in MSVC (with latest of libusb). It is a bit difficult to spot a crash at close as usually is silent but you can see it running it from MSVC (see below) or in a debugger. It does not happen always but frequent enough.

After investigation of the libusb logs and experimentation I think this crash can be avoided by 1) defining zerotv in rtlsdr_read_async as 1 ms instead of zero (see libusb/libusb#1043) or 2) include a short call to Sleep to allow async operations to finish. Please find here a pull request for the latter for your consideration.

Thanks for your great work on the rtlsdr library and kind regards.

Found 1 device(s):
  0:  Realtek, RTL2838UHIDIR, SN: 7

Using device 0: Generic RTL2832U OEM
Found Rafael Micro R820T tuner
[R82XX] PLL not locked!
Sampling at 1536000 S/s.
Tuned to 162000000 Hz.
Tuner gain set to automatic.
Reading samples in async mode...
Signal caught, exiting!

User cancel, exiting...

(process 3988) exited with code -1073741819.

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