Don't crash if PNG has bogus resolution information.

This is a crasher found in [[en:Late_18th_century]] which included
[[File:Frederick-II-of-Prussia-Coloured-drawing.png]] whose metadata
included:

$ identify -verbose foo.png
Image: foo.png
  Format: PNG (Portable Network Graphics)
  Mime type: image/png
  Class: DirectClass
  Geometry: 600x837+0+0
  Resolution: 11837.8x6.3
  Print size: 0.0506851x132.857
  Units: PixelsPerCentimeter
  Type: TrueColorAlpha
  Endianess: Undefined

The bogus "Resolution" data caused a "! Dimension too large." error from
XeLaTeX.

Change-Id: Ic2d48359a568cafa73be16d0ff728ca0a2e97b14