VPP-74 Fix signedness issue when terminal resizes

VPP-74 Fix signedness issue when terminal resizes

When re-locating our current viewport into the pager buffer we need to
verify that the new viewport is within the boundaries of the index.
This condition is considered very rare, but nontheless the check is needed.

Unfortunately I assumed the variable was signed; it is not, and the
subtraction can in some cases cause the value to be negative. This is
therefore a bonafide semantic error that may cause problems.

This patch reworks the logic to avoid having to change it to be signed.

Change-Id: I26f0747d38dcc43dd9c092d50f2489b122009e7b
Signed-off-by: Chris Luke <chrisy@flirble.org>