tracker-miner-fs: Avoid multiple calls to extractor_process_failsafe()
tracker-miner-fs: Avoid multiple calls to extractor_process_failsafe()

The accounting of currently processed files has been also improved, all
to ensure extractor_process_failsafe() is called just once, and tasks
waiting for process_file_cb() don't sneak in and break failsafe
extraction.

outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker