fix another constexpr crash

Author: nunoplopes

llvm_util/llvm2alive.cpp

@@ -1583,6 +1583,7 @@ class llvm2alive_ : public llvm::InstVisitor<llvm2alive_, unique_ptr<Instr>> {
     // If there is a global variable with initializer, put them at init block.
     auto &entry_name = Fn.getFirstBB().getName();
     BB = &Fn.getBB("#init", true);
+    insert_constexpr_before = nullptr;
 
     // Ensure all src globals exist in target as well
     for (auto &gvname : gvnamesInSrc) {