shared/hfp: Don't try read uninitialized data in ringbuffer
shared/hfp: Don't try read uninitialized data in ringbuffer

Don't try to read uninitialized value if '\r' was not found.
This is call stack from test-hfp run with valgrind:

/hfp/test_fragmented_1: ==24869== Conditional jump or move depends on uninitialised value(s)
==24869==    at 0x400A5E4: memchr (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==24869==    by 0x804D24D: can_read_data (hfp.c:359)
==24869==    by 0x804A6E2: read_callback (io-glib.c:168)
==24869==    by 0x4108BFD5: ??? (in /usr/lib/libglib-2.0.so.0.3600.4)
==24869==    by 0x410470E5: g_main_context_dispatch (in /usr/lib/libglib-2.0.so.0.3600.4)
==24869==    by 0x41047497: ??? (in /usr/lib/libglib-2.0.so.0.3600.4)
==24869==    by 0x41047912: g_main_loop_run (in /usr/lib/libglib-2.0.so.0.3600.4)
==24869==    by 0x8049EE0: execute_context (test-hfp.c:186)
==24869==    by 0x4106CBC1: ??? (in /usr/lib/libglib-2.0.so.0.3600.4)
==24869==    by 0x4106CD5B: ??? (in /usr/lib/libglib-2.0.so.0.3600.4)
==24869==    by 0x4106D0E0: g_test_run_suite (in /usr/lib/libglib-2.0.so.0.3600.4)
==24869==    by 0x4106D13C: g_test_run (in /usr/lib/libglib-2.0.so.0.3600.4)

pkarasev3/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,ComputeCycles/bluez,mapfau/bluez,silent-snowman/bluez,pkarasev3/bluez,silent-snowman/bluez,ComputeCycles/bluez,pkarasev3/bluez,silent-snowman/bluez,mapfau/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,mapfau/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,mapfau/bluez