Do not crash or assert on SVG font fallbacks

Fonts falling back to SVG fonts may not have a rendering context, so
just skip kerning in this case.

Task-number: QTBUG-38356
Change-Id: I9d1473bbb8517bfae927e7b020a9c2102c6497b4
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
[mips] Fix branchTruncateDoubleToUint32 implementation in macro assembler.

Test dfg-uint32array-overflow-values fails without this patch.

Change-Id: Ia186ab5c3fbc19fd7621f5cc60678b82a6424641
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>