fix minor problem in previous strtod non-nearest rounding bug fix

fix minor problem in previous strtod non-nearest rounding bug fix

commit 6ffdc4579ffb34f4aab69ab4c081badabc7c0a9a set lnz in the code
path for non-zero digits after a huge string of zeros, but the
assignment of dc to lnz truncates if the value of dc does not fit in
int; this is possible for some pathologically long inputs, either via
strings on 64-bit systems or via scanf-family functions.

instead, simply set lnz to match the point at which we add the
artificial trailing 1 bit to simulate nonzero digits after a huge
run of zeros.