Throw InvalidStateError when calling suspend while rendering has started

This is slightly different from the spec, where it says we should throw
if the suspend time is less than or equal to the current time. However,
this is more or less the same because rendering happens so fast the user
does not even know about the precise 'current time' anyway.

Author: orottier

src/context/offline.rs

@@ -222,7 +222,12 @@ impl OfflineAudioContext {
         // We are mixing async with a std Mutex, so be sure not to `await` while the lock is held
         {
             let mut lock = self.renderer.lock().unwrap();
-            let renderer = lock.as_mut().unwrap();
+            let renderer = match lock.as_mut() {
+                Some(r) => r,
+                None => {
+                    panic!("InvalidStateError: cannot suspend when rendering has already started")
+                }
+            };
 
             match renderer.suspend_promises.entry(quantum) {
                 Entry::Occupied(_) => panic!(
@@ -283,7 +288,10 @@ impl OfflineAudioContext {
             .ceil() as usize;
 
         let mut lock = self.renderer.lock().unwrap();
-        let renderer = lock.as_mut().unwrap();
+        let renderer = match lock.as_mut() {
+            Some(r) => r,
+            None => panic!("InvalidStateError: cannot suspend when rendering has already started"),
+        };
         match renderer.suspend_callbacks.entry(quantum) {
             Entry::Occupied(_) => panic!(
                 "InvalidStateError: cannot suspend multiple times at the same render quantum"