KUDU-2336. diagnostics_log: fix an assertion failure for null stack frames
KUDU-2336. diagnostics_log: fix an assertion failure for null stack frames

On my Ubuntu 16 box, in ASAN builds, it seems I sometimes get a stack
frame with a nullptr address. This caused SymbolSet::Add(...) to fail an
assertion since it uses google::dense_hash_set<> which requires that
there be one key reserved as a non-insertable "empty" marker. We were
using nullptr for that but not checking before potentially inserting it.

This adds a simple workaround. Without the patch, if I started a tserver
with --diagnostics-log-stack-traces-interval-ms=10, it would crash about
10% of the time with the following assertion failure:

kudu-master: ../../thirdparty/installed/common/include/sparsehash/internal/densehashtable.h:969:
  std::pair<iterator, bool> google::dense_hashtable<...>...:
  Assertion `!equals(std::forward<K>(key), key_info.empty_key) && "Inserting the empty key"' failed.

Change-Id: I73ada54bef056b665c03ca142a3995ae6ad59230
Reviewed-on: http://gerrit.cloudera.org:8080/9591
Tested-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

cloudera/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,cloudera/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,InspurUSA/kudu