Fix assertion that can fail on sst corruption (#6780)

Summary:
An assertion that a char == a CompressionType (unsigned char)
originally cast from a char can fail if the original value is negative,
due to numeric promotion.  The assertion should pass even if the value
is invalid CompressionType, because the callee
UncompressBlockContentsForCompressionType checks for that and reports
status appropriately.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6780

Test Plan:
Temporarily change kZSTD = 0x88 and see tests fail. Make this
change (in addition), and tests pass.

Reviewed By: siying

Differential Revision: D21328498

Pulled By: pdillinger

fbshipit-source-id: 61caf8d815581ce49261ecb7ab0f396e9ac4bb92

table/format.cc

@@ -463,7 +463,7 @@ Status UncompressBlockContents(const UncompressionInfo& uncompression_info,
                                const ImmutableCFOptions& ioptions,
                                MemoryAllocator* allocator) {
   assert(data[n] != kNoCompression);
-  assert(data[n] == uncompression_info.type());
+  assert(data[n] == static_cast<char>(uncompression_info.type()));
   return UncompressBlockContentsForCompressionType(uncompression_info, data, n,
                                                    contents, format_version,
                                                    ioptions, allocator);