fix carry into uninitialized slots during printf floating point rounding

fix carry into uninitialized slots during printf floating point rounding

in cases where rounding caused a carry, the slot into which the carry
was taking place was unconditionally treated as valid, despite the
possibility that it could be a new slot prior to the beginning of the
existing non-rounded number. in theory this could lead to unbounded
runaway carry, but in order for that to happen, the whole
uninitialized buffer would need to have been pre-filled with 32-bit
integer values greater than or equal to 999999999.

patch based on proposed fix by Morten Welinder, who also discovered
and reported the bug.

(cherry picked from commit 109048e031f39fbb370211fde44ababf6c04c8fb)