SIGSEGV fix

Problem was identified on Slackware 14.1 x86-64. When you exit from any tool using libr_core.so.0.9.8.git, program crashes with Segmentation fault on stage of deinitialization plugins. Point of the error is that, as shown below, sets the wrong free-function for plist, which later passed the wrong data.

Solution is to clean up the list after the next deinitialization of plugin in r_core_plugin_deinit, then calmly removed the empty list in r_list_free.

I agree, code is not elegant, but do not be angry, examine radare2 sources second day.