fix nits and add new error code for invalid external sequences

lib/zstd_errors.h

@@ -94,6 +94,7 @@ typedef enum {
   ZSTD_error_dstBuffer_wrong     = 104,
   ZSTD_error_srcBuffer_wrong     = 105,
   ZSTD_error_externalMatchFinder_failed = 106,
+  ZSTD_error_invalid_external_sequences = 107,
   ZSTD_error_maxCode = 120  /* never EVER use this value directly, it can change in future versions! Use ZSTD_isError() instead */
 } ZSTD_ErrorCode;