Turn the compression_type check in BlobDBImpl::DecompressSlice into an assertion (#7127)

Summary:
In both cases where `BlobDBImpl::DecompressSlice` is called,
`compression_type` is already checked at the call site; thus, the check
inside the method is redundant and can be turned into an assertion.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7127

Test Plan: `make check`

Reviewed By: zhichao-cao

Differential Revision: D22533454

Pulled By: ltamasi

fbshipit-source-id: ae524443fc6abe0a5fb12327a3fe761a9cd2c831

utilities/blob_db/blob_db_impl.cc

@@ -1148,13 +1148,13 @@ Slice BlobDBImpl::GetCompressedSlice(const Slice& raw,
 Status BlobDBImpl::DecompressSlice(const Slice& compressed_value,
                                    CompressionType compression_type,
                                    PinnableSlice* value_output) const {
-  if (compression_type != kNoCompression) {
+  assert(compression_type != kNoCompression);
+
   BlockContents contents;
   auto cfh = static_cast<ColumnFamilyHandleImpl*>(DefaultColumnFamily());
 
   {
-      StopWatch decompression_sw(env_, statistics_,
+    StopWatch decompression_sw(env_, statistics_, BLOB_DB_DECOMPRESSION_MICROS);
-                                 BLOB_DB_DECOMPRESSION_MICROS);
     UncompressionContext context(compression_type);
     UncompressionInfo info(context, UncompressionDict::GetEmptyDict(),
                            compression_type);
@@ -1165,8 +1165,9 @@ Status BlobDBImpl::DecompressSlice(const Slice& compressed_value,
       return Status::Corruption("Unable to decompress blob.");
     }
   }
+
   value_output->PinSelf(contents.data);
-  }
+
   return Status::OK();
 }