Image management, bug fix

[Miika Turkia]

Hello Guido,

there is buffer overflow situation with this patch as you concatenate
two strings of arbitrary length to a fixed size array. I suggest
validating the lengths and making sure the end result fits to the
target buffer. You should probably also use PATH_MAX for the size
instead of the smaller hard coded size of the new_filename.

A side note / trick; it is slightly confusing that the patch is 5/5.
You could create it relative to current head if the patch is not in a
series e.g. git format-patch HEAD~

miika