Fix uninitialized values in require.c

This was reported running cppcheck over require's code.
require-ess/src/require.c:1002:33: error: Uninitialized variable: &modulediroffs [uninitvar]
               dirname, module, &modulediroffs);
                                ^
require-ess/src/require.c:996:18: note: Assuming condition is false
      if (dirlen == 0) continue; /* ignore empty driverpath elements */
                 ^
require-ess/src/require.c:1002:33: note: Uninitialized variable: &modulediroffs
               dirname, module, &modulediroffs);
                                ^