Patch 7.0.192

Patch 7.0.192
Problem:    When 'swapfile' is switched off in an empty file it is possible
            that not all blocks are loaded into memory, causing ml_get errors
            later.
Solution:   Rename "dont_release" to "mf_dont_release" and also use it to
            avoid using the cached line and locked block.
Files:      src/globals.h, src/memfile.c, src/memline.c

git-svn-id: https://vim.svn.sourceforge.net/svnroot/vim/vim7@209 2a77ed30-b011-0410-a7ad-c7884a0aa172