Guard fp16 renorm shift against zero inputs

[bbopen]

Fixes undefined behavior in fp16_ieee_to_fp32_bits (and the ARM alt variant) where __builtin_clz/_BitScanReverse were invoked with zero for half-precision zeros.

Repro:

cat <<'EOS' >/tmp/fp16_clz_ub.c
#include <stdint.h>
#include <fp16/fp16.h>

int main(void) {
  (void)fp16_ieee_to_fp32_bits(0x0000);  // +0.0h
  return 0;
}
EOS

Before the patch: UBSan reports "passing zero to clz()". After the patch: program runs cleanly.