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GH-41154: [C++] Fix Valgrind error in string-to-float16 conversion (#41155)

### Rationale for this change

Only do the final conversion to float16 on success, to avoid conditional jump on uninitialized values.

Note: this is a benign error. But we want our Valgrind CI job to be as successful as possible.

### Are these changes tested?

Yes, on CI.

### Are there any user-facing changes?

No.
* GitHub Issue: #41154

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>