ext4: Avoid crashing on NULL ptr dereference on a filesystem error

ext4: Avoid crashing on NULL ptr dereference on a filesystem error

If the EOFBLOCK_FL flag is set when it should not be and the inode is
zero length, then eh_entries is zero, and ex is NULL, so dereferencing
ex to print ex->ee_block causes a kernel OOPS in
ext4_ext_map_blocks().

On top of that, the error message which is printed isn't very helpful.
So we fix this by printing something more explanatory which doesn't
involve trying to print ex->ee_block.

Addresses-Google-Bug: #2655740

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>