Fix modification of string literal by swprintf

[Thomas Preudhomme]

Don't over-read memory returned by _DTOA_R, and never write to it
since the result might be a string literal.

For example, when doing:
   swprintf(tt, 20, L"%.*f", 6, 0.0);

we will get back "0".

Instead, write the result returned by _DTOA_R to the output buffer.
After this, write the 0 chars directly to the the output buffer
(if there are any). This also has the (marginal) advantage that
we read/write less memory overall.

The patch, contributed by Silviu Baranga was tested against libcxx testsuite and 
showed no regression. Please find the patch in git format-patch format in 
attachment.

Best regards,

Thomas
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