MP 0.72 search fails if using image base path

[CameronD]

... 'base path' should not be changed after scanning folders.

It is a simple sql one-liner to update any old folders where the basepath has already been stored in full in pathnames.

At a minimum, users should be warned that there are nn folders containing the new base path and this may cause problems later.
They could be given options to

1. do nothing, or
2. update the list to incorporate the base path into the existing folders.

But this should be left for a future improvement, because it is is not part of this search bug.