[InstructionCombining] Fixed null check after dereferencing warning. NFCI.

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -1724,6 +1724,7 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
 
             // The first two arguments can vary for any GEP, the rest have to be
             // static for struct slots
+            assert(CurTy && "No current type?");
             if (J > 1 && CurTy->isStructTy())
               return nullptr;