lib_nbgl: fix uint16_t underflow in page content layout

Found by SDK fuzzing (UBSan implicit-signed-integer-truncation):
availableHeight is a uint16_t decremented by the height each list item consumes.
When content is taller than the screen it underflows to a large value,
defeating the `availableHeight > 10` guard meant
to stop drawing separation lines near the bottom of the container.

Make availableHeight signed (int) in addContent() and
nbgl_pageDrawGenericContentExt(); the > 10 comparisons stay correct.