[patch,libgfortran] Handle invalid command line in EXECUTE_COMMAND_LINE

[FX]

> Otherwise looks good, but strncat is C99, and we support targets which
> don't have a C99 libc (been there, done that..). Since in this case
> you're dealing with string literals rather than user input, it ought
> to be safe to just use plain strcat (or strlen+memcpy, if you prefer).

Nope, strncat() is C90: http://clc-wiki.net/wiki/strncat
Thus, safe to use, I think. I committed as submitted, as revision 227105.

Thanks for the review.
FX