Fix potential divide by zero in display_html

When using logwrapper, there is only a zygote heap. This caused
a divide by zero for the app heap.

Test: native_heapdump_viewer.py --html art/native_heap.txt > test.html

Change-Id: I7defd94935d72ee9c4cb6e53d7a1a308a1030eb3
1 file changed