Fixes an off-by-one gcc 10 warns about, but older gcc's don't.

Note that this issue has been fixed in JDK 11 and later with

8140594: "Various minor code improvements (compiler)"

however, that patch is a lot larger and would need more time to review. Here, I aim to just fix this one case of a broken build such that we can build JDK 8 with GCC 10.