helper: configuration: check for empty search dirs

helper: configuration: check for empty search dirs

The function find_file() is supposed to be called when the search
dirs in 'script_search_dirs' has already been populated.

This is not the case when the command 'ocd_find' is used in one of
the embedded scripts 'startup.tcl'. It then triggers SIGSEGV.

Check for 'script_search_dirs' and eventually skip searching in
the dirs.

Change-Id: I9e75a8739c94de72041fb64487910d60dffcb2bd
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8931
Tested-by: jenkins