justify: initialize a variable before making use of its value

When the mark is off, initialize not just 'bot_x' but also 'top_x'.

This fixes https://savannah.gnu.org/bugs/?55898.

Bug existed since the justifying of a marked region was introduced,
four days ago, with commit f7f5514e.

src/text.c

@@ -2115,6 +2115,7 @@ void do_justify(bool full_justify)
 		/* Start out at the first line of the paragraph. */
 		first_par_line = openfile->current;
 		last_par_line = openfile->current;
+		top_x = 0;
 
 		/* Set the number of lines to be pulled into the cutbuffer. */
 		if (full_justify)