Clam 2612 Fix DatabaseCustomURL CVD prune bug, and add 'valhalla' optional database #2214

Job	Run time
check (libclamav, (iana_cctld\|bytecode_api_\|bytecode_hooks\|rijndael\|yara\|inffixed\|inflate\|queue\|n...	22s
check (libfreshclam)	20s
check (clamav-milter)	21s
check (clambc)	19s
check (clamconf)	20s
check (clamd)	21s
check (clamdscan)	20s
check (clamdtop)	20s
check (clamonacc, (c-thread-pool\|fts\|priv_fts))	22s
check (clamscan)	22s
check (clamsubmit)	18s
check (freshclam)	21s
check (libfreshclam)	21s
check (common)	20s
check (sigtool)	19s
check (examples)	22s
check (win32/compat)	35s
	6m 3s

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