[Ngspice-bugs] Fix incorrect use of strtod

[Marian B. <mar...@po...>]

Hi,

it appears that ngspice assumes that strtod behaves exactly as glibc's
implementation does: Not reporting invalid input with `errno == EINVAL`, but
keeping `errno` unmodified.

The attached patch fixes the use of strtod so that it should work as expected
on both glibc based systems and musl based systems.

See https://www.openwall.com/lists/musl/2024/02/29/13 for a reasoning why musl's
implementation of strtod is POSIX compliant and valid.

Please consider applying the attached patch (or a different fix).

Kind regards,
Marian