imagehlp: improve last error handling in ImageGetCertificateData

[Juan Lang]

Hi Aric,

-    if( !Certificate )
+    if( *RequiredLength < size )
     {
         *RequiredLength = size;
-        return TRUE;
+        SetLastError( ERROR_INSUFFICIENT_BUFFER );
+        return FALSE;
     }

-    if( *RequiredLength < size )
+    if( !Certificate )
     {
         *RequiredLength = size;
-        SetLastError( ERROR_INSUFFICIENT_BUFFER );
-        return FALSE;
+        return TRUE;
     }

Are you sure this is the order that *RequiredLength and Certificate
are checked in?  Seems backwards to the normal style to me.  I won't
bug you for a proper regression test, as I failed to produce one too
:)
--Juan