Fix clang analyzer error (#5924)

Summary:
Without this PR, clang analyzer complains.
```
$USE_CLANG=1 make analyze
db/compaction/compaction_job_test.cc:161:20: warning: The left operand of '==' is a garbage value
      if (key.type == kTypeBlobIndex) {
                ~~~~~~~~ ^
                1 warning generated.
```

Test Plan (on devserver)
```
$USE_CLANG=1 make analyze
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5924

Differential Revision: D17923226

Pulled By: riversand963

fbshipit-source-id: 9d1eb769b5e0de7cb3d89dc90d1cfa895db7fdc8

db/compaction/compaction_job_test.cc

@@ -140,7 +140,7 @@ class CompactionJobTest : public testing::Test {
       std::string skey;
       std::string value;
       std::tie(skey, value) = kv;
-      ParseInternalKey(skey, &key);
+      bool parsed = ParseInternalKey(skey, &key);
 
       smallest_seqno = std::min(smallest_seqno, key.sequence);
       largest_seqno = std::max(largest_seqno, key.sequence);
@@ -158,7 +158,7 @@ class CompactionJobTest : public testing::Test {
 
       first_key = false;
 
-      if (key.type == kTypeBlobIndex) {
+      if (parsed && key.type == kTypeBlobIndex) {
         BlobIndex blob_index;
         const Status s = blob_index.DecodeFrom(value);
         if (!s.ok()) {