config: Explicitly check for bad bit when parsing config line (MR 157)

In gcc 12, the return value for the operator>> of istringstream doesn't seem to indicate success or not, this may have been taking advantage of some undocumented or deprecated behavior... Anyways, with this change the bad bit is checked explicitly after expanding the line.

This also simplifies the code slightly by removing the error var, which was not really necessary.

Fixes: #143 (closed) Replaces: !157 (closed)