conf: fix NULL-pointer dereference in ConfGetInt

conf: fix NULL-pointer dereference in ConfGetInt

If there are empty values in the config-file where integer values are expected, strtoimax in the ConfGetInt-function will segfault because of NULL-pointer dereference.

Here is a configuration example:

pcre.match-limit: []

This will let suricata crash with a segfault.
ASAN-output:

ASAN:DEADLYSIGNAL =================================================================
16951ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7fa690e3ccc5 bp 0x000000000000 sp 0x7ffd0d770ad0 T0)
0 0x7fa690e3ccc4 (/lib/x86_64-linux-gnu/libc.so.6+0x36cc4)
1 0x7fa6946a6534 in strtoimax (/usr/lib/x86_64-linux-gnu/libasan.so.3+0x44534)
2 0x55e0aeba6499 in ConfGetInt /root/suricata-1/src/conf.c:390
3 0x55e0aed2545d in DetectPcreRegister /root/suricata-1/src/detect-pcre.c:99
4 0x55e0aec1b4ce in SigTableSetup /root/suricata-1/src/detect.c:3783
5 0x55e0aeeed58d in PostConfLoadedSetup /root/suricata-1/src/suricata.c:2690
6 0x55e0aeeee4f2 in main /root/suricata-1/src/suricata.c:2892
7 0x7fa690e262b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0)
8 0x55e0aea92d39 in _start (/usr/local/bin/suricata+0xc7d39)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/lib/x86_64-linux-gnu/libc.so.6+0x36cc4)

This commit fixes Ticket #2275