imgdiff: Fix an edge case that leads to infinite loop.

When the input image ends with the magic value sequence of 0x1f, 0x8b,
0x0b (optionally with 0x00), the image parsing code will be stuck in an
infinite loop.

Test: recovery_component_test passes.
Change-Id: Ie3629dfdc41360387b19cc3e0359c95ae4fb998e