linux: silence uninitialized variable warning

When building using gyp and BUILDTYPE=Release using clang 3.4 received
this warning:

../src/unix/linux-core.c:640:34: warning: variable 'n' is uninitialized
      for (len = sizeof("cpu0"); n /= 10; len++);
                                 ^

Initializing n = 0 silences this build warning.