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.

Affected function: wolfSSH_ClearScreen.
Issue: F-48

Author: ejohnstown

src/wolfterm.c

@@ -119,10 +119,7 @@ static void wolfSSH_ClearScreen(WOLFSSH_HANDLE handle, word32 x1, word32 y1, wor
             fill = x2 - x1;
         }
         else { /* | y1 - y2 | * maxX - x1 + x2 */
-            fill = y1 - y2;
-            if (fill < 0)
-                fill += fill * 2;
-            fill = fill * maxX - x1 + x2;
+            fill = ((y1 > y2) ? y1 - y2 : y2 - y1) * maxX - x1 + x2;
         }
 
         FillConsoleOutputCharacterA(handle, ' ', fill, start, &w);