Guard against impossible tile coordinates when decoding

decode.cpp

@@ -119,6 +119,12 @@ void handle(std::string message, int z, unsigned x, unsigned y, int describe, st
 			}
 		}
 
+		// X and Y are unsigned, so no need to check <0
+		if (x > (1 << z) || y > (1 << z)) {
+			fprintf(stderr, "Impossible tile %d/%u/%u\n", z, x, y);
+			exit(EXIT_FAILURE);
+		}
+
 		layer_to_geojson(stdout, layer, z, x, y, !pipeline, pipeline, pipeline, 0, 0, 0, !force);
 
 		if (!pipeline) {