pack.c: avoid returning uninitialized String

	Fix unpacking with 'b', 'B', 'h' and 'H' format. Do not return an
	uninitialized String to Ruby before filling the content bytes.
	Fixes r11175 ("pack.c (pack_unpack): execute block if given with
	unpacked value instead of creating an array", 2006-10-15).
	[ruby-core:78841] [Bug #13075]
	test/ruby/test_pack.rb: fix test case added by r57187

	The test case for String#unpack added by r57187 is not properly testing
	because the String will be filled after the block invocation.
	[ruby-core:78841] [Bug #13075]

	Thanks to nagachika for pointing this out:

	  http://d.hatena.ne.jp/nagachika/20161226/ruby_trunk_changes_57184_57194#r57187

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@58088 b2dd03c8-39d4-4d8f-98ff-823fe69b080e