Fix integer-to-bit-string conversions to handle the first fractional byte

Fix integer-to-bit-string conversions to handle the first fractional byte
correctly when the output bit width is wider than the given integer by
something other than a multiple of 8 bits.

This has been wrong since I first wrote that code for 8.0 :-(.  Kudos to
Roman Kononov for being the first to notice, though I didn't use his
patch.  Per bug #5237.