libroot_build _kern_read_dir(): reset errno before readdir()

Since we're checking errno when readdir() didn't return an entry, we
need to reset it before, or we might see a spurious error value.

src/build/libroot/fs.cpp

@@ -609,6 +609,7 @@ _kern_read_dir(int fd, struct dirent *buffer, size_t bufferSize,
 
 	// get the next entry
 	dirent *entry;
+	errno = 0;
 	if (dynamic_cast<AttrDirDescriptor*>(descriptor))
 		entry = fs_read_attr_dir(descriptor->dir);
 	else