No functional change.
- 0 -> NULL
- Don't use cast.
- Style fix.
- Fix typos in comment.
In the exp2_values test case, provide separate expected return values
for the float case, reflecting the actual exp2f() argument value after
rounding to float precision.  Fixes PR lib/49256.  Thanks to Makoto
Kamada and Tetsuya Isaki for the analysis.