Ensure functions always have a return even in release builds.

src/wren_compiler.c

@@ -2258,6 +2258,7 @@ static int getNumArguments(const uint8_t* bytecode, const Value* constants,
 
     default:
       UNREACHABLE();
+      return 0;
   }
 }
 

src/wren_value.c

@@ -668,7 +668,8 @@ ObjClass* wrenGetClass(WrenVM* vm, Value value)
   }
   #endif
 
-  return NULL; // Unreachable.
+  UNREACHABLE();
+  return NULL;
 }
 
 bool wrenValuesEqual(Value a, Value b)

src/wren_vm.c

@@ -1007,10 +1007,12 @@ static bool runInterpreter(WrenVM* vm)
       // A CODE_END should always be preceded by a CODE_RETURN. If we get here,
       // the compiler generated wrong code.
       UNREACHABLE();
-      DISPATCH();
   }
 
-  ASSERT(0, "Should not reach end of interpret.");
+  // We should only exit this function from an explicit return from CODE_RETURN
+  // or a runtime error.
+  UNREACHABLE();
+  return false;
 }
 
 WrenInterpretResult wrenInterpret(WrenVM* vm, const char* sourcePath,