elf: Check if PT_INTERP entry exists

Not every ELF file has a PT_INTERP enty. This commit will catch a
potential AttributeError if no PT_INTERP entry is found.

Before this patch:
```
viper xxx > elf interp
[!] The command elf raised an exception:
Traceback (most recent call last):
  File "/home/vault/Code/viper/viper/core/ui/console.py", line 201, in start
    module.run()
  File "/home/vault/Code/viper/modules/elf.py", line 156, in run
    self.interp()
  File "/home/vault/Code/viper/modules/elf.py", line 108, in interp
    print("Program interpreter: {0}".format(interp.get_interp_name()))
AttributeError: 'NoneType' object has no attribute 'get_interp_name'
```

After:
```
viper xxx > elf interp
No PT_INTERP entry found
```