fpexif: Minor optimization.

git-svn-id: https://svn.code.sf.net/p/lazarus-ccr/svn@8642 8e941d3f-bd1b-0410-a28a-d453659cc2b4

components/fpexif/fpemetadata.pas

@@ -529,12 +529,12 @@ var
   sa: ansistring;
  {$ENDIF}
 begin
-  p := AStream.Position;
   streamsize := AStream.Size;
 
   if not ((ReadByte(AStream) = $FF) and (ReadByte(AStream) = M_SOI)) then
     exit;
 
+  p := AStream.Position;
   while p < streamsize do begin
     // The basic structure of the jpeg segments is
     //   $FF ..... identifier (sometimes repeated)