Undefined behaviour crash on NaN/inf/-inf input

Sorry another UB bug:

I'm aware that zfp doesn't currently support round-tripping non-finite values, but it would be good if it didn't crash hard on NaN/inf/-inf input, in particular since client libraries like xarray / zarr will often pad data with NaNs, sometimes at a fairly low level in the library e.g. to achieve uniform chunk sizes before writing to zarr.

As in https://github.com/LLNL/zfp/issues/241, with clang I get SIGILL crashes, and with UBSAN enabled it's attributed to, e.g.:

third_party/zfp/src/template/encodef.c:57:17: runtime error: nan is outside the range of representable values of type 'int'
    #0 0x55cf45e3edf7 in zfp_encode_block_float_4 third_party/zfp/src/template/encodef.c:98
    #1 0x55cf45e3f916 in zfp_encode_partial_block_strided_float_4 third_party/zfp/src/template/encode4.c:88:10
    #2 0x55cf45e480bc in compress_strided_float_4 third_party/zfp/src/template/compress.c:105:13
    #3 0x55cf45e464b2 in zfp_compress third_party/zfp/src/zfp.c:1116:3
...
SUMMARY: UndefinedBehaviorSanitizer: float-cast-overflow third_party/zfp/src/template/encodef.c:57:17

Assuming we want to allow some default conversion to happen silently, this can be worked around via -fno-strict-float-cast-overflow -fno-sanitize=float-cast-overflow, but I'm reluctant to enable these too widely in case it masks some other bug.

Start by reproducing the NaN, inf, and -inf cases with UBSAN, then inspect third_party/zfp/src/template/encodef.c around line 57 and the callers in encode4.c and compress.c. Confirm the behavior through zfp_compress and define completion as avoiding the reported float-cast-overflow and SIGILL crashes, with the intended handling of non-finite values agreed before implementation.