Avoid warning thrash

...between "error: unannotated fall-through between switch labels
[-Werror,-Wimplicit-fallthrough]" (in some builds, for the original code) and
"error: fallthrough annotation in unreachable code
[-Werror,-Wimplicit-fallthrough]" (in other builds, after adding
SAL_FALLTHROUGH).

Change-Id: I75bbefd69c81f43f22117f8ad110237de24e18af

sal/osl/unx/file_error_transl.cxx

@@ -164,9 +164,8 @@ oslFileError oslTranslateFileError(int Errno)
         case ETIMEDOUT:
             return osl_File_E_TIMEDOUT;
 
-        case 0:
-            assert(false);
         default:
+            assert(Errno != 0);
             /* FIXME translateFileError: is this alright? Or add a new one: osl_File_E_Unknown? */
             return osl_File_E_invalidError;
     }