FLONUM-TO-DIGITS handles non-negative input properly

FLONUM-TO-DIGITS handles non-negative input properly

 lp#308961, part 1.

 SBCL has an extended version of the Burger & Dybwig fp printer, which
 supports rounding.

 It did not however support zero -- but in eg.

   (format nil "~,1F" 0.001)

 0.001 is rounded to zero, which leads FLONUM-TO-DIGITS giving us one zero too
 many. Ie. it should be

    => "0.0"

 but prior to this we got "0.00" instead.

 This patch removes the special casing for 0, and instead tests that the lower
 limit of the interval (- r m-) is positive in order not to generate extra
 zeros.