Fix tab completion segfault in reverse search

If I happen to hit the tab key while in reverse search mode Julia crashes with the error message
```
(reverse-i-search)`': ERROR: `input_string` has no method matching input_string(::SearchState)
 in complete_line at REPL.jl:294
 in complete_line at LineEdit.jl:988
 in complete_line at LineEdit.jl:139
 in anonymous at LineEdit.jl:1100
 in prompt! at ./LineEdit.jl:1397
 in run_interface at ./LineEdit.jl:1372
 in run_interface at /Users/andreasnoackjensen/julia/usr/lib/julia/sys.dylib
 in run_frontend at ./REPL.jl:819
 in run_repl at ./REPL.jl:170
 in _start at ./client.jl:399
 in _start at /Users/andreasnoackjensen/julia/usr/lib/julia/sys.dylib
```
@jakebolewski proposed this change, which seems to fix the problem.
cc: @keno