Coverity static analysis reports that the code that scans the queue
families for one that has the graphics bit, can be -1 if none are
found, which could cause a problem when the -1 (signed) value is
used later as an index in a uint32_t (unsigned) variable.
Its not immediately clear how often this could occur, not finding
a queue family with the graphics bit, but adding some protecting
just in case.
