replacing: do not try to wipe nonexistent multidata, to avoid crashing

This fixes https://savannah.gnu.org/bugs/?58970.
Reported-by: Matteo Raso <mraso@uoguelph.ca>

Bug existed since version 5.0, commit 569d0056.

src/search.c

@@ -493,6 +493,7 @@ char *replace_line(const char *needle)
 void wipe_and_recalculate_colorinfo(void)
 {
 	for (linestruct *line = openfile->filetop; line != NULL; line = line->next)
+		if (line->multidata)
 			for (short index = 0; index < openfile->syntax->nmultis; index++)
 				line->multidata[index] = -1;