Fix test_bz2.py

Fix some corner cases in the irgen refcounter, and fix a refcount
issue in the bz2 module itself.

from_cpython/Modules/bz2module.c

@@ -1420,6 +1420,9 @@ BZ2File_init(BZ2FileObject *self, PyObject *args, PyObject *kwargs)
 
     mode = (mode_char == 'r') ? "rb" : "wb";
 
+    // Pyston change: this prevents leaking refs in the test_bz2.py test
+    Py_CLEAR(self->file);
+
     self->file = PyObject_CallFunction((PyObject*)&PyFile_Type, "(Osi)",
                                        name, mode, buffering);
     if (self->file == NULL)

src/codegen/irgen/refcounts.cpp

@@ -100,7 +100,7 @@ bool RefcountTracker::isNullable(llvm::Value* v) {
 }
 
 void RefcountTracker::refConsumed(llvm::Value* v, llvm::Instruction* inst) {
-    if (llvm::isa<UndefValue>(v))
+    if (llvm::isa<UndefValue>(v) || llvm::isa<ConstantPointerNull>(v))
         return;
 
     assert(this->vars[v].reftype != RefType::UNKNOWN);
@@ -109,7 +109,7 @@ void RefcountTracker::refConsumed(llvm::Value* v, llvm::Instruction* inst) {
 }
 
 void RefcountTracker::refUsed(llvm::Value* v, llvm::Instruction* inst) {
-    if (llvm::isa<UndefValue>(v))
+    if (llvm::isa<UndefValue>(v) || llvm::isa<ConstantPointerNull>(v))
         return;
 
     assert(this->vars[v].reftype != RefType::UNKNOWN);