Add an error message when PC is invalid

Closes #160

pwndbg/commands/nearpc.py

@@ -9,6 +9,7 @@ import gdb
 from capstone import *
 
 import pwndbg.arguments
+import pwndbg.color
 import pwndbg.color.context as C
 import pwndbg.color.disasm as D
 import pwndbg.color.nearpc as N
@@ -55,6 +56,10 @@ def nearpc(pc=None, lines=None, to_string=False, emulate=False):
     pc    = int(pc)
     lines = int(lines)
 
+    # Check whether we can even read this address
+    if not pwndbg.memory.peek(address):
+        return [pwndbg.color.red('Invalid address %#x' % address)]
+
     # # Load source data if it's available
     # pc_to_linenos = collections.defaultdict(lambda: [])
     # lineno_to_src = {}