Always fill in root table length

Otherwise, there may be DCHECK failure in JitCodeCache::ClearData.
Fixes gcstress JIT tests.

Test: test/run-test --host --relocate --prebuild  --64 --jit --gcstress --dev 021-string2

Change-Id: I757ccd1e5a015c5e686254417a2a3f68b650da1c
1 file changed