Fixed error when trying to read keyboard when there are no chars ready

src/main.cpp

@@ -489,9 +489,13 @@ class Emulator {
 
 				// Input
 				case 0x1E: {
-					uint8_t c = e->input.front();
-					e->input.pop();
-					return c;
+					if (e->input.size()) {
+						uint8_t c = e->input.front();
+						e->input.pop();
+						return c;
+					} else {
+						return 0x00;
+					}
 				}
 
 				// Has input