Fix crash after a tab was opened during reload.
Commit
0b99fa70ca9d0f976655e61adee1a5eebacc0734 "Bug 620: Reset form
fields to default values on reload" made render_document() decrement
vs->form_info_len to 0 while vs->form_info remained non-NULL.
copy_vs() then copied the whole structure with copy_struct and did not
change form_info because form_info_len was 0. Both view_state
structures had form_info pointing to the same memory block, causing a
segfault when destroy_vs() tried to free that block a second time.
Reported by أحمد المحمودي.