strtod: work around IRIX 6.5 bug

strtod: work around IRIX 6.5 bug

IRIX mis-parses "1e 1" as 10.0 and "" instead of 1.0 and "e 1".
Because the original parse may differ from the reparse in terms
of whether the value overflows, we have to do an errno dance.

* lib/strtod.c (strtod): Reparse number on shorter string if
exponent parse was invalid.
* tests/test-strtod.c (main): Add check for "0x1p 2".
Reported by Tom G. Christensen.

Signed-off-by: Eric Blake <eblake@redhat.com>