RFR(S): 8251918: [Graal] Crash in DumpTimeSharedClassInfo::add_verification_constraint

[calvin.cheung at oracle.com]

bug: https://bugs.openjdk.java.net/browse/JDK-8251918

webrev: http://cr.openjdk.java.net/~ccheung/jdk16/8251918/webrev.00/

After the fix for JDK-8247529, the 
SystemDictionaryShared::find_or_allocate_info_for(InstanceKlass* k) 
could return NULL if dynamic CDS dump is in progress.

The proposed fix is to do a NULL check on the return value of 
SystemDictionaryShared::find_or_allocate_info_for().

Testing: Tier1 - 4, hs-tier4-graal, hs-tier8-graal

thanks,

Calvin