namespace: use ferror() to detect errors of fscanf()

Currently we check errno, but musl-libc doesn't zero it,
if fscanf() has returned EOF without errors.

travis-ci: success for A few fixes for Alpine Linux
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/namespaces.c

@@ -750,8 +750,8 @@ static int parse_id_map(pid_t pid, char *name, UidGidExtent ***pb_exts)
 		ret = fscanf(f, "%d %d %d", &ext->first,
 				&ext->lower_first, &ext->count);
 		if (ret != 3) {
-			if (errno != 0) {
-				pr_perror("Unable to parse extents");
+			if (ferror(f)) {
+				pr_perror("Unable to parse extents: %d", ret);
 				ret = -1;
 			} else
 				ret = 0;