Clam 2612 Fix DatabaseCustomURL CVD prune bug, and add 'valhalla' optional database #2215

Job	Run time
check (libclamav, (iana_cctld\|bytecode_api_\|bytecode_hooks\|rijndael\|yara\|inffixed\|inflate\|queue\|n...	19s
check (libfreshclam)	20s
check (clamav-milter)	21s
check (clambc)	21s
check (clamconf)	23s
check (clamd)	21s
check (clamdscan)	21s
check (clamdtop)	25s
check (clamonacc, (c-thread-pool\|fts\|priv_fts))	20s
check (clamscan)	18s
check (clamsubmit)	19s
check (freshclam)	20s
check (libfreshclam)	22s
check (common)	19s
check (sigtool)	22s
check (examples)	19s
check (win32/compat)	22s
	5m 52s

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