signature: avoids overflow from VariableNameHash

[catenacyber]

Link to redmine ticket:
None

Describe changes:

• Avoids a read heap buffer overflow due to VariableNameHash not limiting its hash size

Found by fuzzing SigInit

Stack trace is following

==12==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x62d000030400 at pc 0x000000ae8bc6 bp 0x7ffd627f4a30 sp 0x7ffd627f4a28
READ of size 8 at 0x62d000030400 thread T0 (Suricata-Main)
SCARINESS: 23 (8-byte-read-heap-buffer-overflow)
#0 0xae8bc5 in HashListTableAdd /src/suricata/src/util-hashlist.c:135:9
#1 0xb9f0d5 in VariableNameGetIdx /src/suricata/src/util-var-name.c:193:9
#2 0xb9ed4f in VarNameStoreSetupAdd /src/suricata/src/util-var-name.c:324:10
#3 0x7e7f27 in DetectFlowbitSetup /src/suricata/src/detect-flowbits.c:249:15
#4 0x855dad in SigParseOptions /src/suricata/src/detect-parse.c:742:13
#5 0x84df20 in SigParse /src/suricata/src/detect-parse.c:1174:19
#6 0x84a55a in SigInitHelper /src/suricata/src/detect-parse.c:1798:9
#7 0x849fbe in SigInit /src/suricata/src/detect-parse.c:1944:16