fix NoneType access if emulator not available through import (#166)

This fixes a regression when the emulator support is disabled via
setting it to None when the import fails. The access happens when
checking if a particular architecture actually supports emulation.

Ensure that emulate has the correct value depending on the availability
of the import and deactivate it otherwise. The following core can safely
rely on the boolean state of the emulate variable.

pwndbg/disasm/__init__.py

@@ -172,14 +172,14 @@ def near(address, instructions=1, emulate=False):
     insns.append(current)
 
     # Some architecture aren't emulated yet
-    if pwndbg.arch.current not in pwndbg.emu.emulator.arch_to_UC:
+    if not pwndbg.emu or pwndbg.arch.current not in pwndbg.emu.emulator.arch_to_UC:
         emulate = False
 
     # Emulate forward if we are at the current instruction.
     emu = None
 
     # If we hit the current instruction, we can do emulation going forward from there.
-    if address == pc and pwndbg.emu and emulate:
+    if address == pc and emulate:
         emu = pwndbg.emu.emulator.Emulator()
 
         # For whatever reason, the first instruction is emulated twice.