Remove undefined behavior caught by LLVM 10 UBSAN.

[Gaelan]

LLVM 10's UBSAN catches several cases of undefined behavior when running the test suite:

• jv_get cast array indices to an int, even if the index is out of range. I added a bounds check.
• Several places do pointer math on null pointers, which is undefined behavior. I fixed this by special casing null.

Additionally, I added -fno-sanitize-recover=undefined to the CFLAGS, which is necessary to make UBSAN actually crash the program (and therefore fail the tests) instead of just logging an error.

[coveralls]

Coverage increased (+0.01%) to 84.139% when pulling 5dc92fe on Gaelan:llvm10-ubsan into ccc79e5 on stedolan:master.