Handle the empty register list

Close #277

Author: cyrus-and

.gdbinit

@@ -1985,6 +1985,10 @@ class Registers(Dashboard.Module):
             changed = self.table and (self.table.get(name, '') != string_value)
             self.table[name] = string_value
             registers.append((name, string_value, changed))
+        # handle the empty register list
+        if not registers:
+            msg = 'No registers to show (check the "dashboard registers -style list" attribute)'
+            return [ansi(msg, R.style_error)]
         # compute lengths considering an extra space between and around the
         # entries (hence the +2 and term_width - 1)
         max_name = max(len(name) for name, _, _ in registers)
@@ -2036,7 +2040,8 @@ class Registers(Dashboard.Module):
             'list': {
                 'doc': '''String of space-separated register names to display.
 
-The empty list (default) causes to show all the available registers.''',
+The empty list (default) causes to show all the available registers. For
+architectures different from x86 setting this attribute might be mandatory.''',
                 'default': '',
                 'name': 'register_list',
             }