wmcalc: Null-termitate display string when clearing.

Often, the number currently being displayed has more than 10 digits.
However, when clearing the display, only the first 10 digits were cleared.
When a new number was entered, it would be prepended to any digits that
weren't cleared.  This behavior was invisible to the user, causing
incorrect calculations, e.g., Debian bug #564173:

1814 / 720 * 300 -> 7568.41111

But the correct answer is 755.833333.

The problem was that when the display was cleared, the string that stores
this number was not null-terminated.  Indeed, 1814 / 720 gives us
2.519444444 (6 4's), but only 2.51944444 (5 4's) is displayed.  When we
begin to multiply by 300, the final 4 remained, and so we really were
multiplying by 3004.

wmcalc/wmcalcfunc.c

@@ -63,6 +63,7 @@ void clearcalc(void) {
   for (i=0; i < DISPSIZE; i++) {
     DispString[i] = ' ';
   }
+  DispString[DISPSIZE] = '\0';
   OpFlag = ' ';
 } /***** End of function clearcalc() *****************************/
 
@@ -94,6 +95,7 @@ void clearnum(void) {
   for (i=0; i < DISPSIZE; i++) {
     DispString[i] = ' ';
   }
+  DispString[DISPSIZE] = '\0';
 
 } /***** End of function clearnum() ******************************/