Prevent panic in calls to mineral_type() when labs are empty (#305)

CHANGELOG.md

@@ -113,6 +113,8 @@ Unreleased
 - Change `pathfinder::search_many` to return an incomplete result when called with no goals to
   prevent a panic due to unexpected return data from javascript.
 - Change `MemoryReference::get` to return a generic error type
+- Change `StructureLab::mineral_type` to return `Option<ResourceType>`, avoiding panic when labs
+  are empty
 
 ### Misc:
 

src/objects/impls/structure_lab.rs

@@ -1,11 +1,25 @@
+use stdweb::Value;
+
 use crate::{
     constants::{ResourceType, ReturnCode},
     objects::{Creep, StructureLab},
+    traits::TryFrom,
 };
 
 impl StructureLab {
-    pub fn mineral_type(&self) -> ResourceType {
-        js_unwrap!(__resource_type_str_to_num(@{self.as_ref()}.mineralType))
+    pub fn mineral_type(&self) -> Option<ResourceType> {
+        let mineral_v = js! {
+            const mineral = @{self.as_ref()}.mineralType;
+            if (mineral) {
+                return __resource_type_str_to_num(mineral);
+            }
+        };
+        match mineral_v {
+            Value::Number(_) => {
+                Some(ResourceType::try_from(mineral_v).expect("lab resource unknown."))
+            }
+            _ => None,
+        }
     }
 
     pub fn boost_creep(&self, creep: &Creep, body_part_count: Option<u32>) -> ReturnCode {