Initialize out parameters to NULL.

Fixes a crash in the debugger, where we were freeing random pointers:

(lldb)
frame #2: 0x000dc981 monotouchtest`method_commands_internal(command=<unavailable>, method=<unavailable>, domain=<unavailable>, p=0x7f52aca4, end=<unavailable>, buf=<unavailable>) + 9009 at debugger-agent.c:8116
   8113			g_free (il_offsets);
   8114			g_free (line_numbers);
   8115			g_free (column_numbers);
-> 8116			g_free (end_line_numbers);
   8117			g_free (end_column_numbers);
   8118			g_free (source_files);
   8119			g_ptr_array_free (source_file_list, TRUE);