vcard_emul_rsa_op: assert pad_len >= 3

After the assert, there is a pad_len -= 3; statement.
Since pad_len should not be negative, when calling memset, make sure it's 3.

This fixes the following gcc warning when building vcard_emul_nss.c:
In file included from /usr/include/string.h:519,
                 from /usr/include/glib-2.0/glib/gslice.h:26,
                 from /usr/include/glib-2.0/glib.h:79,
                 from ../src/vcard_emul_nss.c:14:
In function ‘memset’,
    inlined from ‘vcard_emul_rsa_op.part.0’ at ../src/vcard_emul_nss.c:369:5:
/usr/include/bits/string_fortified.h:71:10: warning: ‘memset’ specified \
size between 18446744071562067968 and 18446744073709551615 exceeds \
maximum object size 9223372036854775807 [-Wstringop-overflow=]
   71 |   return __builtin___memset_chk (__dest, __ch, __len, __bos0 (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Uri Lublin <uril@redhat.com>
12 jobs for fix_pad_len_assert in 8 minutes and 31 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Build
passed CentOS 7 build #4880437

00:01:33

passed Fedora build #4880438

00:02:30

passed Fedora clang #4880439

00:02:36

passed Fedora meson #4880447

00:03:10

passed Ubuntu build #4880436

00:01:14

failed Fedora meson #4880435

00:02:01

 
  Test
passed CentOS 7 build and check #4880440

00:01:53

passed Fedora asan #4880444

00:02:42

passed Fedora build and check #4880442

00:02:43

passed Fedora csbuild #4880445

00:02:23

passed Fedora memcheck #4880443

00:02:16

passed Ubuntu build and check #4880441

00:01:12