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 8 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #3490609
nettle3 linux
Build and Check - clang

31:11

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

37:58

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

30:04

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

16:45

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

33:02

passed #3490614
freebsd
Build and Check on FreeBSD

44:07

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

30:05

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

27:09

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

23:43

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

06:11

passed #3490603
nettle3 linux
Compile minimal library and openssl compat

08:02

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

02:17:33

69.7%