Fix build with Clang 4.0

--- src/FDM/JSBSim/FGFDMExec.cpp.orig	2017-01-20 21:38:08.784722000 +0000
+++ src/FDM/JSBSim/FGFDMExec.cpp	2017-01-20 22:55:24.501656000 +0000
@@ -211,7 +211,7 @@
   
   SetGroundCallback(0);
 
-  if (FDMctr > 0) (*FDMctr)--;
+  if (FDMctr != 0) (*FDMctr)--;
 
   Debug(1);
 }