osc: bug: uneven clipboard bytes where not buffered correctly

When responding to a OSC 52 clipboard request, we need to base64
encode the clipboard data.

This is done in, potentially, several calls. Since we need at least 3
bytes to be able to produce any base64 output, we may have to buffer
up to 2 bytes between the callback calls with clipboard data.

This was being done incorrectly, where both bytes were written to
index 0 in the buffer.

osc.c

@@ -111,10 +111,12 @@ from_clipboard_cb(const char *text, size_t size, void *user)
     if (left == 0)
         return;
 
+    assert(ctx->idx == 0);
+
     int remaining = left % 3;
     for (int i = remaining; i > 0; i--)
-        ctx->buf[0] = text[size - i];
-    ctx->idx = remaining;
+        ctx->buf[ctx->idx++] = text[size - i];
+    assert(ctx->idx == remaining);
 
     char *chunk = base64_encode((const uint8_t *)t, left / 3 * 3);
     assert(chunk != NULL);