Unsigned variable compared < 0

When filling the screen with spaces, the code was subtracting two
unsigned numbers and checking if they were negative. Changed to use a
comparison and adjust the subtraction as appropriate, then did the rest
of the size expansion. If the second point is before the first, set the
fill length to 0.

Affected function: wolfSSH_ClearScreen.
Issue: F-48

Author: ejohnstown

src/wolfterm.c

@@ -115,14 +115,15 @@ static void wolfSSH_ClearScreen(WOLFSSH_HANDLE handle, word32 x1, word32 y1, wor
         start.Y = y1;
 
         /* get number of cells  */
-        if (y1 == y2) { /* on same line so is x2 - x1 */
-            fill = x2 - x1;
+        if (y2 == y1) { /* on same line so is x2 - x1 */
+            fill = (x2 >= x1) ? (x2 - x1) : 0;
         }
-        else { /* | y1 - y2 | * maxX - x1 + x2 */
-            fill = y1 - y2;
-            if (fill < 0)
-                fill += fill * 2;
-            fill = fill * maxX - x1 + x2;
+        /* (y2 - y1) * maxX - x1 + x2 */
+        else if (y2 > y1) {
+            fill = (y2 - y1) * maxX - x1 + x2;
+        }
+        else {
+            fill = 0;
         }
 
         FillConsoleOutputCharacterA(handle, ' ', fill, start, &w);