format: flonum-to-string: Fix corner-case when printing "0.0".

When printing 0.0 and fdigits parameter set to 0 or nil, single "."
was printed, what is not valid float number. Now it at least prints
".0".

Signed-off-by: Daniel Kochmański <dkochmanski@turtle-solutions.eu>

src/lsp/format.lsp

@@ -99,11 +99,11 @@
   (declare (type float x))
   (cond ((zerop x)
          ;; Zero is a special case which FLOAT-STRING cannot handle.
-         (if fdigits
+         (if (and fdigits (/= fdigits 0))
              (let ((s (make-string (1+ fdigits) :initial-element #\0)))
                (setf (schar s 0) #\.)
-               (values s (length s) t (zerop fdigits) 0))
-             (values "." 1 t t 0)))
+               (values s (length s) t nil 0))
+             (values ".0" 2 t nil 0)))
         (t
          (multiple-value-bind (e string)
              (cond