bug#18932: #18932 string->number errors on e.g. "1e400xyz"

[guile@discard.email]

Note that guile 2.0.11 also gives strange results for large negative exponents:
scheme@(guile-user)> (string->number "1e-400")ERROR: In procedure 
string->number:ERROR: In procedure string->number: Value out of range: -400
 
For comparison, in Scheme 48 1.9
(string->number "1e-400") => 0.0
(string->number "1e400") => +inf.0
(string->number "1e400xyz") => #f