Fix repaintable stack calculation (esp8266#5821)

* Fix repaintable stack calculation

Fixes esp8266#5794 as found by @mattbradford83

* Overwrite last word of stack as well

Under-by-one error would not reset the absolute end of the stack, adjust
comparison to fix.

cores/esp8266/cont_util.cpp

@@ -73,12 +73,10 @@ void cont_repaint_stack(cont_t *cont)
     register uint32_t *sp asm("a1");
     // Ensure 64 bytes adjacent to the current SP don't get touched to endure
     // we don't accidentally trounce over locals or IRQ temps.
-    uint32_t sp_safe = CONT_STACKSIZE/4 - ((sp - &cont->stack[0] - 64)/4);
-
     // Fill stack with magic values
-    for(uint32_t pos = 0; pos < sp_safe; pos++)
+    for ( uint32_t *pos = sp - 16; pos >= &cont->stack[0]; pos-- )
     {
-        cont->stack[pos] = CONT_STACKGUARD;
+       *pos = CONT_STACKGUARD;
     }
 }