simpleperf: fix small errors in JITDebugReader.

1. When seeing an entry with symfile_size = 0, we should move
   to the next entry.
2. Skip labels having zero length in jit debug file.

Bug: none
Test: run simpleperf_unit_test.
Change-Id: Iaa914cc4c6567ae5950194507c82e92316802a80
1 file changed