signed integer overflow in unit tests crashes hardened build on 32-bit trusty

If I build on gcc with --enable-fragile-hardening on ubuntu trusty, I get a crash in dir/download_status_random_backoff :

#0  0xf7fd9d70 in __kernel_vsyscall ()
#1  0xf6599827 in raise () from /lib/i386-linux-gnu/libc.so.6
#2  0xf659cc53 in abort () from /lib/i386-linux-gnu/libc.so.6
#3  0x56e5e16c in __addvsi3 ()
#4  0x5679e366 in download_status_random_backoff_helper (
    min_delay=min_delay@entry=0, max_delay=max_delay@entry=2147483647)
    at src/test/test_dir.c:4167
#5  0x5679e699 in test_dir_download_status_random_backoff (arg=0x0)
    at src/test/test_dir.c:4196
#6  0x56a519dc in testcase_run_bare_ (
    testcase=testcase@entry=0x5724e0c0 <dir_tests+640>)
    at src/ext/tinytest.c:106
#7  0x56a51de1 in testcase_run_one (group=<optimized out>, 
    group@entry=0x572346b0 <testgroups+208>, testcase=<optimized out>, 
    testcase@entry=0x5724e0c0 <dir_tests+640>) at src/ext/tinytest.c:253
#8  0x56a532c4 in tinytest_main (c=2, v=0xffffd764, 
    groups=0x572345e0 <testgroups>) at src/ext/tinytest.c:435
#9  0x5661eede in main (c=2, v=<optimized out>)
    at src/test/testing_common.c:319

I think this has something to do with our fixes for #17750 (moved) or #20534 (moved), but I'm not certain.

I have a patch that fixes this issue for me.