[gfortran,patch] Fix PR27588: Add substring-out-of-bounds check

[FX Coudert]

> FX wrote (regarding his patch): "I've been working to find how to  
> output
> the variable name, [...] but it still doesn't work in all cases."
> If you find an example where it does not work, I'm happy to look at  
> it.
> (I didn't find one, but I didn't look for it extensively)

Looking for one is easy: simply add a gcc_assert(name) inside  
gfc_conv_substring (but outside the "if (flag_bounds_check)" block),  
rebuild and run the testsuite: if there are new failures, then you  
have your case you can (happily) look into it ;-)

Even if you don't have time to do it, I'd suggest opening a PR with  
the restults of the procedure (it can also be applied on the array  
bounds checking code).

Regards,
FX