gdb Pretty Print: generic encoded was failing on reference/pointer types
If you debug this program using **gdb**
```rust
fn main() {
let x = 10;
let y = Some(&x);
// additional code
}
```
And you try to print **y**'s value from the debugger, you get the following:
```
(gdb) print y
Python Exception <class 'gdb.error'> Cannot convert value to int.:
$1 = {RUST$ENCODED$ENUM$0$None = Some = {0x7fff5fbff97c}}
```
What happens is that inside **debugger_pretty_printers_common.py** the method `is_null_variant` doesn't have any special handling for pointer values so it ends up calling `.as_integer()` on `discriminant_val` (which holds a pointer) and fails.
Considering it needs to handle pointers and return _true_ when the pointer is _null_, I modified the `.as_integer()` method in **gdb_rust_pretty_printing.py** to take pointers into consideration.
After this modification **gdb** prints **y** like this:
```
(gdb) print y
$1 = Some = {0x7fff5fbff97c}
```
Now, it would be nice to print something useful (instead of a pointer address) but the pretty printer doesn't currently handle references/pointers so that's a completely different subject.
b358319353