No-suffix mod_rewrite dynamic album image handling

Dynamic album images with the mod_rewrite option turned on *without* a
mod_rewrite suffix added weren't working: the target image link of
DYNAMIC/IMAGE didn't work because only 'DYNAMIC/IMAGE.alb' was checked
for existance, but 'DYNAMIC.alb' needs to be checked as well.
Merge pull request #449 from jagerman/master

Fix mod_rewrite dynamic album image handling when not using a rewrite suffix (issue #447)