Merge pull request #32 from omarandlorraine/stackfix

Stackfix

src/cpu.rs

@@ -1361,5 +1361,11 @@ mod tests {
                 }
             }
         }
+	}
+
+	#[test]
+	fn stack_underflow() {
+		let mut cpu = CPU::new();
+		let _val: u8 = cpu.pull_from_stack();
     }
 }

src/registers.rs

@@ -154,11 +154,11 @@ impl StackPointer {
     // JAM: FIXME: Should we prevent overflow here? What would a 6502 do?
 
     pub fn decrement(&mut self) {
-        self.0 -= 1;
+		self.0 = self.0.wrapping_sub(1);
     }
 
     pub fn increment(&mut self) {
-        self.0 += 1;
+		self.0 = self.0.wrapping_add(1);
     }
 }