truncate: Clamp file size to 0 when using the -s option

When using the `-s` option to reduce the size of a file, the file size
is now set to zero if the argument given would result in a file
size less than zero.

This matches the behavior of truncate on Linux and FreeBSD.

Userland/Utilities/truncate.cpp

@@ -99,7 +99,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         size = stat.st_size + size;
         break;
     case OP_Shrink:
-        size = stat.st_size - size;
+        size = max(stat.st_size - size, 0);
         break;
     }