fix gzip related issues

Author: Grahame Grieve

dependencies/zflate/zflate.pas

@@ -79,7 +79,7 @@   tgzipinfo = record
 
 var
   zchunkmaxsize: dword = 1024*128; //128 KB default max chunk size
-  zbuffersize: dword = 1024*1024*4; //4 MB default buffer size
+  zbuffersize: dword = 1024*1024*16; //16 MB default buffer size
 
 threadvar
   zlasterror: integer;

dependencies/zflate/zflatefiles.pas

@@ -56,7 +56,7 @@ function gzip(bytes : TBytes; header : boolean; level: dword=9) : TBytes;
 function ungzip(bytes : TBytes) : TBytes;
 begin
   result := zflate.zdecompress(bytes);
-  if length(result) = 0 then
+  if zlastError <> 0 then
     raise EFslException.create('Failed to read compressed content: '+zflatetranslatecode(zlasterror));
 end;