memory: prevent a leak when copying the leading quoting to the next line

This fixes https://savannah.gnu.org/bugs/?60596.

Bug existed since version 4.4, commit 8fce33af from two years ago,
since this automatic copying of the quoting part was introduced.

src/text.c

@@ -1333,6 +1333,7 @@ bool do_wrap(void)
 
 		openfile->current_x += lead_len;
 #ifndef NANO_TINY
+		free(openfile->undotop->strdata);
 		update_undo(ENTER);
 #endif
 		if (autowhite)