android/client: Fix uninitialized "sock_fd" variable

If EXEC() macro is called with the first pointer argument NULL, sock_fd
will not be initialized. Given that the NULL check is not fatal, it is a
good idea to initialize the variable to -1 so the code has defined
behavior on this situation.

Detected by clang:

android/client/if-sock.c:251:7: error: variable 'sock_fd' is used
uninitialized whenever 'if' condition is false
[-Werror,-Wsometimes-uninitialized]