Don't preload into nonexistent zygote

We tried to access the first entry of Build.SUPPORTED_32_BIT_ABIS
even when there where none, causing an out of bounds array access.
The resulting exception was handled correctly, so this only
speeds things up a little and avoids log spam.

Bug: 228390898
Test: Build and boot master, Treehugger
Change-Id: I5827c6991065dd50d3f6e797930a9e6b7616de6e