Disable options_settable_test in UBSAN and fix UBSAN failure in blob_…

Summary:
…db_test

options_settable_test won't pass UBSAN so disable it.
blob_db_test fails in UBSAN as SnapshotList doesn't initialize all the fields in dummy snapshot. Fix it. I don't understand why only blob_db_test fails though.
Closes https://github.com/facebook/rocksdb/pull/3477

Differential Revision: D6928681

Pulled By: siying

fbshipit-source-id: e31dd300fcdecdfd4f6af279a0987fd0cdec5122

Makefile

@@ -527,6 +527,10 @@ PARALLEL_TEST = \
 	transaction_test \
 	write_prepared_transaction_test \
 
+# options_settable_test doesn't pass with UBSAN as we use hack in the test
+ifdef COMPILE_WITH_UBSAN
+        TESTS := $(shell echo $(TESTS) | sed 's/\boptions_settable_test\b//g')
+endif
 SUBSET := $(TESTS)
 ifdef ROCKSDBTESTS_START
         SUBSET := $(shell echo $(SUBSET) | sed 's/^.*$(ROCKSDBTESTS_START)/$(ROCKSDBTESTS_START)/')

db/snapshot_impl.h

@@ -45,6 +45,10 @@ class SnapshotList {
     list_.prev_ = &list_;
     list_.next_ = &list_;
     list_.number_ = 0xFFFFFFFFL;      // placeholder marker, for debugging
+    // Set all the variables to make UBSAN happy.
+    list_.list_ = nullptr;
+    list_.unix_time_ = 0;
+    list_.is_write_conflict_boundary_ = false;
     count_ = 0;
   }