ui: fix crash due to out-of-bounds array access

If the search string typed at the "Find file:" prompt in the file finder
does not match any files, then the resulting file list is empty. When
ENTER is pressed, open_file_finder() attempts to retrieve the
non-existent entry at index 0 in the empty list, often resulting in a
crash. Fix this boundary condition error.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Jonas Fonseca <jonas.fonseca@gmail.com>

src/ui.c

@@ -293,7 +293,7 @@ open_file_finder(const char *commit)
 	finder.keymap = get_keymap("search", STRING_SIZE("search")),
 	file_finder_update(&finder);
 	file_finder_draw(&finder);
-	if (read_prompt_incremental("Find file: ", FALSE, file_finder_input_handler, &finder))
+	if (read_prompt_incremental("Find file: ", FALSE, file_finder_input_handler, &finder) && finder.pos.lineno < finder.lines)
 		file = get_path(finder.line[finder.pos.lineno]->text);
 
 	file_finder_done(&finder);