[bug] Fix FSE_readNCount()

* Fix bug introduced in PR #2271
* Fix long-standing bug that is impossible to trigger inside of zstd
* Add a fuzzer that makes sure the normalized count always round trips
  correctly

tests/fuzz/fuzz.py

@@ -60,6 +60,7 @@ TARGET_INFO = {
     'raw_dictionary_round_trip': TargetInfo(InputType.RAW_DATA),
     'dictionary_stream_round_trip': TargetInfo(InputType.RAW_DATA),
     'decompress_dstSize_tooSmall': TargetInfo(InputType.RAW_DATA),
+    'fse_read_ncount': TargetInfo(InputType.RAW_DATA),
 }
 TARGETS = list(TARGET_INFO.keys())
 ALL_TARGETS = TARGETS + ['all']