llvm: fix is_func_defined
llvm: fix is_func_defined

CallInst::getCalledFunction can return even nullptr
and in that case we got segfault

Signed-off-by: Marek Chalupa <a05df156a69ad59faa9af9b666ef78009594c145@gmail.com>

mchalupa/dg,mchalupa/dg,mchalupa/dg,mchalupa/dg