Fix build errors.

memory/ram.cpp

@@ -4,7 +4,7 @@ RAM::RAM(u16 size) : size(size), readonly(false) {
   memory = new u8[size];
 }
 
-RAM::RAM(u8* memory, u16 size, bool readonly = false)
+RAM::RAM(u8* memory, u16 size, bool readonly)
   : memory(memory), size(size), readonly(readonly)
 {}
 
@@ -25,7 +25,7 @@ void RAM::write16(u16 addr, u16 data) {
 }
 
 u16 RAM::read16(u16 addr) {
-  if(addr >= size - 1) return;
+  if(addr >= size - 1) return 0xFFFFu;
   u16 *ptr = (u16*)&memory[addr];
   return *ptr;
 }