Fix segfault when p cmd is mistyped

This commit changes this behavior:

```
$ r2 /bin/true 
 -- Invert the block bytes using the 'I' key in visual mode
[0x00008e80]> ps 0x08049434
Block size 134517812 is too big
Segmentation fault
```

For this one:

```
$ r2 /bin/true 
 -- Execute commands on a temporally offset appending '@ offset' to your command
[0x00401124]> ps 0x08049434
Block size 134517812 is too big
This block size is too big. Did you mean 'ps @ 0x08049434' instead?
[0x00401124]>
```

The comment line removal is because that TODO seems to be done.

As a side comment, that block of code (before the switch statement) is kind of messy, specially with var assignments.

Kind!