Stack: Fix null-dereference READ
Stack: Fix null-dereference READ

Fixes: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=13573

Credit to oss-fuzz

jpfr/open62541,jpfr/open62541,open62541/open62541,open62541/open62541,jpfr/open62541,StalderT/open62541,StalderT/open62541,JGrothoff/open62541,jpfr/open62541,open62541/open62541,StalderT/open62541,JGrothoff/open62541,JGrothoff/open62541,open62541/open62541,JGrothoff/open62541,StalderT/open62541