ocsp: corrected the comparison of the serial size in OCSP response

Previously the OCSP certificate check wouldn't verify the serial length
and could succeed in cases it shouldn't.

Reported by Stefan Buehler.
12 jobs from master in 220 minutes and 8 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #3490609
nettle3 linux
Build and Check - clang

00:31:11

passed #3490606
x86 lcov nettle3 linux
Build and Check - separate build dir (x86)

00:37:58

69.8%
passed #3490607
lcov nettle3 linux
Build and Check - without SSL 3.0

00:30:04

69.2%
passed #3490612
nettle3 wine mingw linux
Build and Check for windows in separate build dir

00:16:45

passed #3490610
nettle3 linux
Build and Check in FIPS140-2 mode

00:33:02

passed #3490614
freebsd
Build and Check on FreeBSD

00:44:07

passed #3490605
ubsan lcov nettle3 linux
Build and Check with C99 and ubsan

00:30:05

68.7%
passed #3493366
linux asan lcov nettle3
Build and Check with asan

00:27:09

68.5%
passed #3490613
nettle3 lcov linux
Build and Check with valgrind

00:23:43

67.5%
passed #3490608
x86-64 nettle3 abi-checker linux
Build with ABI-check (x86-64)

00:06:11

passed #3490603
nettle3 linux
Compile minimal library and openssl compat

00:08:02

failed #3490604
nettle3 lcov asan linux
Build and Check with asan

02:17:33

69.7%