Fix divide by zero, case when heap is 100% allocated.

Author: mhightower83

cores/esp8266/Esp-frag.cpp

@@ -34,8 +34,13 @@ void EspClass::getHeapStats(uint32_t* hfree, uint16_t* hmax, uint8_t* hfrag)
  *hfree = (uint32_t)ummHeapInfo.freeBlocks * umm_block_size();
  if (hmax)
  *hmax = umm_free_blocks_to_free_space(ummHeapInfo.maxFreeContiguousBlocks);
- if (hfrag)
+ if (hfrag) {
+ if (ummHeapInfo.freeBlocks) {
  *hfrag = 100 - (sqrt32(ummHeapInfo.freeBlocksSquared) * 100) / ummHeapInfo.freeBlocks;
+ } else {
+ *hfrag = 0;
+ }
+ }
 
 }