Handle the case when target disappears instead of reactivating

[monich]

Avoid this use-after-free scenario:

Invalid read of size 8
  at 0x1115AD: nfc_target_reactivate_request_timeout_ms (nfc_target.c:325)
  by 0x112002: nfc_target_submit_request (nfc_target.c:707)
  by 0x1120D7: nfc_target_submit_next_request (nfc_target.c:732)
  by 0x11282C: nfc_target_reactivate (nfc_target.c:1001)
  ...
Address 0x4ec9648 is 24 bytes inside a block of size 64 free'd
  at 0x484B27F: free
  by 0x11174E: nfc_target_reactivate_request_free (nfc_target.c:395)
  by 0x111C59: nfc_target_free_request (nfc_target.c:603)
  by 0x111C95: nfc_target_fail_request (nfc_target.c:614)
  by 0x11223D: nfc_target_fail_requests (nfc_target.c:783)
  by 0x112BDD: nfc_target_gone_handler (nfc_target.c:1152)
  by 0x112B64: nfc_target_gone (nfc_target.c:1114)
  ...
  by 0x111513: nfc_target_reactivate_request_submit (nfc_target.c:302)
  by 0x111FED: nfc_target_submit_request (nfc_target.c:706)
  by 0x1120D7: nfc_target_submit_next_request (nfc_target.c:732)
  by 0x11282C: nfc_target_reactivate (nfc_target.c:1001)
  ...
Block was alloc'd at
  at 0x4848899: malloc
  by 0x11177B: nfc_target_reactivate_request_new (nfc_target.c:420)
  by 0x112805: nfc_target_reactivate (nfc_target.c:997)
  ...

[monich]

Thanks!