re PR libfortran/79540 (FAIL: gfortran.dg/fmt_fw_d.f90 -O0 execution test)

	PR libgfortran/79540
	* io/write_float.def (build_float_string): Don't copy digits when
	ndigits is negative.

From-SVN: r269911

libgfortran/ChangeLog

+2019-03-25  John David Anglin  <danglin@gcc.gnu.org>
+
+	PR libgfortran/79540
+	* io/write_float.def (build_float_string): Don't copy digits when
+	ndigits is negative.
+
 2019-03-05  Jakub Jelinek  <jakub@redhat.com>
 
 	PR libgfortran/89593

libgfortran/io/write_float.def

@@ -620,14 +620,15 @@ build_float_string (st_parameter_dt *dtp, const fnode *f, char *buffer,
     }
 
   /* Set digits after the decimal point, padding with zeros.  */
-  if (nafter > 0)
+  if (ndigits >= 0 && nafter > 0)
     {
       if (nafter > ndigits)
 	i = ndigits;
       else
 	i = nafter;
 
-      memcpy (put, digits, i);
+      if (i > 0)
+	memcpy (put, digits, i);
       while (i < nafter)
 	put[i++] = '0';