libvt: return safe pointers for empty output

Normalize empty buffers and borrowed strings at the libghostty-vt C
boundary to null pointers.

Zig can use sentinel addresses such as 0x1 for empty slices. Returning
these pointers to Go can cause a fatal invalid-pointer error when the
runtime relocates a goroutine's stack, even though the length is zero.

include/ghostty/vt/allocator.h

@@ -224,7 +224,8 @@ typedef struct GhosttyAllocator {
  *
  * @param allocator Pointer to the allocator to use, or NULL for the default
  * @param len Number of bytes to allocate
- * @return Pointer to the allocated buffer, or NULL if allocation failed
+ * @return Pointer to the allocated buffer, or NULL if len is zero or
+ *         allocation failed
  *
  * @ingroup allocator
  */

include/ghostty/vt/formatter.h

@@ -196,6 +196,8 @@ GHOSTTY_API GhosttyResult ghostty_formatter_format_buf(GhosttyFormatter formatte
  * The caller is responsible for freeing the returned buffer with
  * ghostty_free(), passing the same allocator (or NULL for the default)
  * that was used for the allocation.
+ * Empty output returns GHOSTTY_SUCCESS with *out_ptr set to NULL and
+ * *out_len set to zero. This result can be passed to ghostty_free().
  *
  * @param formatter The formatter handle (must not be NULL)
  * @param allocator Pointer to allocator, or NULL to use the default allocator

include/ghostty/vt/selection.h

@@ -907,6 +907,8 @@ GHOSTTY_API GhosttyResult ghostty_terminal_selection_format_buf(
  * The returned buffer is allocated using allocator, or the default allocator
  * if NULL is passed. The caller owns the returned buffer and must free it with
  * ghostty_free(), passing the same allocator and returned length.
+ * Empty output returns GHOSTTY_SUCCESS with *out_ptr set to NULL and
+ * *out_len set to zero. This result can be passed to ghostty_free().
  *
  * The returned bytes are not NUL-terminated. This supports plain text, VT, and
  * HTML uniformly as byte output.

include/ghostty/vt/terminal.h

@@ -2176,7 +2176,8 @@ GHOSTTY_API GhosttyResult ghostty_terminal_continuation_buf(
  * The returned bytes are allocated with allocator, or the default allocator
  * when allocator is NULL. The caller must release them with ghostty_free(),
  * passing the same allocator and returned length. An empty continuation is a
- * successful zero-length allocation.
+ * successful result with *out_ptr set to NULL and *out_len set to zero,
+ * which can also be passed to ghostty_free().
  * Continuation tracking must have been enabled by setting
  * GHOSTTY_TERMINAL_OPT_CONTINUATION_MAX_BYTES to a nonzero value before the
  * input that produced the continuation was written.

include/ghostty/vt/types.h

@@ -262,6 +262,8 @@ typedef enum GHOSTTY_ENUM_TYPED {
  *
  * The memory is not owned by this struct. The pointer is only valid
  * for the lifetime documented by the API that produces or consumes it.
+ * Empty strings produced by the library have a non-NULL pointer to valid
+ * storage.
  */
 typedef struct {
   /** Pointer to the string bytes. */