fix "fin edits" crash

$ ./fin --version
Version 1.510
$ ./fin edits
Recent Edits
Segmentation fault (core dumped)

//dbg sess
Program received signal SIGSEGV, Segmentation fault.
0x0000000000419d72 in show_recent_adjustments (account=0x62da60 "current", no_of_transactions=5,
    column_separator=32 ' ', search_string=0x0, year=0, month_number=0) at src/summary.c:552
    552             if (strlen(search_string)==0) {