[Bug pch/78970] GCC crashes if input file is dash

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78970

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
Created attachment 40455
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40455&action=edit
Untested patch