debugger: Fix crash with GDB 7.7

GDB 7.7 doesn't understand `&` as an argument to `-exec-run`.

Moreover, the code handling errors from GDB has an issue with this
error and leads to a crash.

Closes https://sourceforge.net/p/geany-plugins/bugs/112/

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@cesspit Are you OK with this?  I think it's important to do something as it leads to a crash when using GDB 7.7 (version is according to your findings).  I also think that even if it prevents asynchronous debugging, it's still worth to fix until a proper asynchronous fix is released.