Fix Kernel#singleton_method with Module#Prepend

	* proc.c (rb_obj_singleton_method): search the method entry from
	  the origin class, for fix prepended modules.  [Bug #14658]

	From: Vasiliy Ermolovich <younash@gmail.com>

	proc.c: fix segfault when no singleton class

	* proc.c (rb_obj_singleton_method): bail out if the receiver does
	  not have the singleton class without accessing the origin class
	  not to segfault.  [Bug #14658]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@65118 b2dd03c8-39d4-4d8f-98ff-823fe69b080e