Fix for bug 15570313 "Missing text on nonexistent font file"
This patch makes sure that the lastChar and mInstances arrays are in
sync with each other even when a FontFamily being added has no valid
fonts in it. Previously, when they got out of sync, unicode coverage
calculation would be wrong, resulting in missing text.
Change-Id: I69c727ef69e2c61e2b2d6b81d5a28c806327f865