dir: Fix integer overflow in read_fd on 32-bit platforms

st_size is a 64-bit off_t but was truncated to gsize which is
32-bit on 32-bit platforms. A file larger than G_MAXSIZE - 1 would
cause size + 1 to overflow to 0, leading to a zero-size allocation
followed by an oversized read.

common/flatpak-dir.c

@@ -8172,6 +8172,13 @@ read_fd (int          fd,
   gsize size;
   gsize alloc_size;
 
+  if (stat_buf->st_size < 0 || (guint64) stat_buf->st_size > G_MAXSIZE - 1)
+    {
+      g_set_error_literal (error, G_FILE_ERROR, G_FILE_ERROR_NOMEM,
+                           _("Not enough memory"));
+      return FALSE;
+    }
+
   size = stat_buf->st_size;
 
   alloc_size = size + 1;