selftests/nolibc: prevent out of bounds access in expect_vfprintf

If read() fails and returns -1 (or returns garbage for some other
reason) buf would be accessed out of bounds.
Only use the return value of read() after it has been validated.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>

tools/testing/selftests/nolibc/nolibc-test.c

@@ -1051,7 +1051,6 @@ static int expect_vfprintf(int llen, int c, const char *expected, const char *fm
 	lseek(fd, 0, SEEK_SET);
 
 	r = read(fd, buf, sizeof(buf) - 1);
-	buf[r] = '\0';
 
 	fclose(memfile);
 
@@ -1061,6 +1060,7 @@ static int expect_vfprintf(int llen, int c, const char *expected, const char *fm
 		return 1;
 	}
 
+	buf[r] = '\0';
 	llen += printf(" \"%s\" = \"%s\"", expected, buf);
 	ret = strncmp(expected, buf, c);