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

regress/image-sixel-buffer.sh

@@ -0,0 +1,32 @@
+#!/bin/sh
+
+# Catch a trailing NUL written beyond an exactly full SIXEL output buffer.
+
+PATH=/bin:/usr/bin
+export PATH
+
+SIXEL_TEST_DIR=$(mktemp -d) || exit 1
+trap 'rm -rf "$SIXEL_TEST_DIR"' 0 1 15
+export SIXEL_TEST_DIR
+ASAN_OPTIONS=detect_leaks=0
+export ASAN_OPTIONS
+
+case $(uname -s) in
+Darwin) SIXEL_TEST_LDFLAGS=-Wl,-dead_strip ;;
+*) SIXEL_TEST_LDFLAGS=-Wl,--gc-sections ;;
+esac
+export SIXEL_TEST_LDFLAGS
+
+# Skip when the configured compiler or its runtime cannot use ASan.
+printf 'int main(void) { return 0; }\n' >"$SIXEL_TEST_DIR/probe.c"
+if ! make -s -C .. -f Makefile -f regress/image-sixel-buffer.mk \
+    sixel-buffer-probe >"$SIXEL_TEST_DIR/probe.log" 2>&1; then
+	exit 0
+fi
+if ! "$SIXEL_TEST_DIR/probe" >"$SIXEL_TEST_DIR/probe.log" 2>&1; then
+	exit 0
+fi
+
+make -s -C .. -f Makefile -f regress/image-sixel-buffer.mk \
+    sixel-buffer-test || exit 1
+"$SIXEL_TEST_DIR/test" || exit 1