[libc][NFC] Fix -Wdangling-else when compiling libc with gcc >= 7 (#67833)

Explicit braces were added to fix the "suggest explicit braces to avoid
ambiguous ‘else’" warning since the current solution (switch (0) case 0:
default:) doesn't work since gcc 7 (see
https://github.com/google/googletest/issues/1119)

gcc 13 generates about 5000 of these warnings when building libc without
this patch.