Fix undefined behavior test failures

[skeeto]

The tests currently fail under UBSan:

$ CFLAGS=-fsanitize=undefined LDFLAGS=-fsanitize=undefined cmake -B build
$ cmake --build build
$ build/tests/example_doc_test_v1 2>&1 | grep 'runtime error'
src/tokenizer.c:99:5: runtime error: null pointer passed as argument 2, which is declared to never be null
src/emitter.c:104:20: runtime error: -inf is outside the range of representable values of type 'int'
src/emitter.c:106:9: runtime error: negation of -2147483648 cannot be represented in type 'int'; cast to an unsigned type to negate this value to itself

The specific errors depend on whether you're using GCC or Clang, but that's three instances of undefined behavior. The first comes from passing null to memmove(), which does not accept null pointers (before C2y). My fix tests the length instead of the pointer so that a null pointer with a non-zero length will still trip UBSan rather than hide a bug.

The second is an overflow casting infinity to int, which in practice produces INT_MIN which then overflows abs. I fixed this by not computing the log of zero.

[tjol]

Thanks for this. Sorry I haven't had time for it earlier.