Fix off-by-one buffer growth in sixel_print_add and add regression test.

regress/image-sixel-buffer.c

@@ -0,0 +1,72 @@
+/* Exercise SIXEL output lengths at the encoder's allocation boundary. */
+
+#undef NDEBUG
+#include <assert.h>
+
+#include "../tmux.h"
+
+#ifdef ENABLE_IMAGES
+#include "../image-sixel.c"
+#include "../xmalloc.c"
+
+/* Abort if an allocation or formatting check fails. */
+void
+fatal(__unused const char *fmt, ...)
+{
+	abort();
+}
+
+/* Abort if an allocation or formatting check fails. */
+void
+fatalx(__unused const char *fmt, ...)
+{
+	abort();
+}
+
+/* Encode lengths below, at and above the initial 8192-byte allocation. */
+int
+main(void)
+{
+	struct sixel_image	 image = { 0 };
+	u_int			 colour, width, x;
+	size_t			 size;
+	char			*data;
+
+	colour = (2u << 25) | (100u << 16) | (100u << 8) | 100u;
+	image.sy = 2;
+	image.ncolours = 1;
+	image.used_colours = 1;
+	image.colours = &colour;
+	image.lines = xcalloc(2, sizeof *image.lines);
+
+	/* Headers and controls add 26 bytes to these uncompressed rasters. */
+	for (width = 8165; width <= 8167; width++) {
+		image.sx = width;
+		image.lines[0].sx = width;
+		image.lines[1].sx = width;
+		image.lines[0].pixels = xcalloc(width, sizeof(uint16_t));
+		image.lines[1].pixels = xcalloc(width, sizeof(uint16_t));
+
+		/* Alternate row bits so adjacent columns cannot be compressed. */
+		for (x = 0; x < width; x++)
+			image.lines[x % 2].pixels[x] = 1;
+		data = sixel_print(&image, NULL, &size);
+		assert(data != NULL);
+		assert(size == width + 26);
+		assert(data[size] == '\0');
+		assert(strlen(data) == size);
+		free(data);
+		free(image.lines[0].pixels);
+		free(image.lines[1].pixels);
+	}
+	free(image.lines);
+	return (0);
+}
+#else
+/* Skip the encoder check when image support is disabled. */
+int
+main(void)
+{
+	return (0);
+}
+#endif