bug#16738: Errors while resizing vertically

[Paul Eggert]

My change adjusted set-window-new-pixel so that it rejects attempts to 
set the new pixel size to a negative value. Here, new-pixel is currently 
0, so (set-window-new-pixel window-17 t) is attempting to change 
new-pixel to be -17.

Are negative new-pixel values allowed?  If so, the attached patch should 
fix the problem.  If not, the problem's probably in the caller.

newpixel.diff
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