Fix crashes when input program tried to access undefined struct types.

(This in particular would happen when there was an error in the body of a struct
definition and we were left with an UndefinedStructType and then later tried to
do loads/stores from/to it.)

Issue #356.

ctx.cpp

@@ -2484,6 +2484,12 @@ FunctionEmitContext::LoadInst(llvm::Value *ptr, llvm::Value *mask,
         AssertPos(currentPos, ptrType != NULL);
     }
 
+    if (CastType<UndefinedStructType>(ptrType->GetBaseType())) {
+        Error(currentPos, "Unable to load to undefined struct type \"%s\".",
+              ptrType->GetBaseType()->GetString().c_str());
+        return NULL;
+    }
+
     if (ptrType->IsUniformType()) {
         if (ptrType->IsSlice()) {
             return loadUniformFromSOA(ptr, mask, ptrType, name);
@@ -2964,6 +2970,12 @@ FunctionEmitContext::StoreInst(llvm::Value *value, llvm::Value *ptr,
         AssertPos(currentPos, ptrType != NULL);
     }
 
+    if (CastType<UndefinedStructType>(ptrType->GetBaseType())) {
+        Error(currentPos, "Unable to store to undefined struct type \"%s\".",
+              ptrType->GetBaseType()->GetString().c_str());
+        return;
+    }
+
     // Figure out what kind of store we're doing here
     if (ptrType->IsUniformType()) {
         if (ptrType->IsSlice())