Detect cJump opcodes in FParser AutoDiff

The AutoDiff code silently ignored the unsupported use of an ```if()``` statement and dis not throw an error like it was supposed to. This is a quick fix, but I will have to sit down and figure out why this is not working as intended. 

Without this fix invalid bytecode will be generated by AutoDiff in certain cases. Eval'ing it will result in a segfault. This error has existed for quite a while, but I only uncovered it now, as I test more thoroughly for unsupported derivatives and proper error status return :-/

I'll work on this a bit more tomorrow.