fix disasm.one() to be called without an address (#189)

This brings back the functionality to call disasm.one() without
and target address. As a default value the current regs.pc is
selected.
Fix the disasm.near() call to not pass None to disasm.one() when
the backward cache misses, otherwise it wrongly falls back to
the regs.pc value (which near() is not supposed to do in its
context).

pwndbg/disasm/__init__.py

@@ -101,10 +101,10 @@ def get_one_instruction(address):
         return ins
 
 def one(address=None):
-    if address is None or not pwndbg.memory.peek(address):
-        return None
     if address is None:
         address = pwndbg.regs.pc
+    if not pwndbg.memory.peek(address):
+        return None
     for insn in get(address, 1):
         backward_cache[insn.next] = insn.address
         return insn
@@ -163,10 +163,12 @@ def near(address, instructions=1, emulate=False):
     # before, which were followed by this one.
     needle = address
     insns  = []
-    insn   = one(backward_cache[current.address])
+    cached = backward_cache[current.address]
+    insn   = one(cached) if cached else None
     while insn is not None and len(insns) < instructions:
         insns.append(insn)
-        insn = one(backward_cache[insn.address])
+        cached = backward_cache[insn.address]
+        insn = one(cached) if cached else None
     insns.reverse()
 
     insns.append(current)