shared/hfp: Fix using uninitialized value

shared/hfp: Fix using uninitialized value

This fixes the following trace when running unit tests:
  at 0x409597: is_response (hfp.c:1009)
  by 0x409597: hf_call_prefix_handler (hfp.c:1103)
  by 0x409955: hf_process_input (hfp.c:1176)
  by 0x409955: hf_can_read_data (hfp.c:1240)
  by 0x40C0E8: watch_callback (io-glib.c:170)
  by 0x48C3EDC: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.6000.3)
  by 0x48C426F: ??? (in /usr/lib64/libglib-2.0.so.0.6000.3)
  by 0x48C45A2: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.6000.3)
  by 0x40C94C: mainloop_run (mainloop-glib.c:79)
  by 0x40CEF9: mainloop_run_with_signal (mainloop-notify.c:201)
  by 0x408A43: tester_run (tester.c:870)
  by 0x404E04: main (test-hfp.c:865)

Change-Id: Ie720545383bf5ec748b20afd6b87517fa22bdef8
Signed-off-by: himanshu <h.himanshu@samsung.com>