Removed unwanted Exception (#546)

Removes an exception in the tcache/bins command where the exception would be printed as well as an error message.
This is purely a UI fix as described in Issue #535 (https://github.com/pwndbg/pwndbg/issues/535)

pwndbg/heap/ptmalloc.py

@@ -150,6 +150,7 @@ class Heap(pwndbg.heap.heap.BaseHeap):
             except Exception as e:
                 print(message.error('Error fetching tcache. GDB cannot access '
                                     'thread-local variables unless you compile with -lpthread.'))
+                return None
         else:
             if not self.has_tcache():
                 print(message.warn('Your libc does not use thread cache'))