call-barring: Quiet GCC warning

  CC       src/call-barring.o
In function ‘cb_lock_property_lookup’,
    inlined from ‘cb_set_property’ at src/call-barring.c:854:7:
src/call-barring.c:776:18: error: ‘len’ may be used uninitialized [-Werror=maybe-uninitialized]
  776 |         property += len;
      |         ~~~~~~~~~^~~~~~
src/call-barring.c: In function ‘cb_set_property’:
src/call-barring.c:759:16: note: ‘len’ was declared here
  759 |         size_t len;
      |                ^~~