Fix crashes from malformed programs.

ctx.cpp

@@ -1850,6 +1850,11 @@ llvm::Value *
 FunctionEmitContext::AllocaInst(LLVM_TYPE_CONST llvm::Type *llvmType, 
                                 const char *name, int align, 
                                 bool atEntryBlock) {
+    if (llvmType == NULL) {
+        assert(m->errorCount > 0);
+        return NULL;
+    }
+
     llvm::AllocaInst *inst = NULL;
     if (atEntryBlock) {
         // We usually insert it right before the jump instruction at the

expr.cpp

@@ -3350,11 +3350,13 @@ StructMemberExpr::GetType() const {
         return NULL;
 
     const Type *elementType = structType->GetElementType(identifier);
-    if (elementType == NULL)
+    if (elementType == NULL) {
         Error(identifierPos,
               "Element name \"%s\" not present in struct type \"%s\".%s",
               identifier.c_str(), structType->GetString().c_str(),
               getCandidateNearMatches().c_str());
+        return NULL;
+    }
 
     const PointerType *pt = dynamic_cast<const PointerType *>(expr->GetType());
     if (structType->IsVaryingType() ||