[Bug other/44435] gengtype: don't test undefined value after vasprintf failure

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44435

--- Comment #8 from joseph at codesourcery dot com <joseph at codesourcery dot 
com> ---
I have not been tracking the state of review or lack thereof for these 
patches.