Fix error reporting from wcstof()

When wcstof() skipped initial space and then parsing failed, it set
endptr to the first non-space character. Fix it to correctly report
failure by setting endptr to the beginning of the input string.
The fix is from theraven@, who fixed this bug in wcstod() and
wcstold() in r227753.

While I'm here:

Move assignments out of declarations in wcstod() and wcstold().
This is against my personal preference, but it is our agreed style(9).

Set endptr correctly on malloc() failure in all three functions.

Remove an incorrect comment: This is pointer arithmetic,
so the code was not actually making that assumption.

wcstold() advanced the wcp pointer beyond leading whitespace
and then reset it back to the beginning of the string.
Do not reset it. This seems to have no functional effect,
since strtold_l() also skips leading whitespace. I'm making
the change to keep this function consistent with wcstof() and
wcstod(), and because the C11 spec prescribes the use of iswspace()
to skip leading space.

Reported by: libc++ unit test for std::stof(std::wstring)
MFC after: 8 days
Sponsored by: Dell EMC