ls: always print "?" for allocated size of a dereferenced dangling symlink

ls: always print "?" for allocated size of a dereferenced dangling symlink

Previously for `ls -Ls` (but not `ls -Lsl`), we referenced
the st_blocks returned from the previous failed stat() call.
This undefined value was seen to be 0 for dangling symlinks at least.
* src/ls.c (print_file_name_and_frills, length_of_file_name_and_frills):
Don't use st_blocks if the previous stat() failed
* tests/ls/dangle: Add a test case
* NEWS: Mention the fix, and roll up related items into a single entry.