memory: do not allocate space for multidata when it's already allocated

Allocating it again would leak the existing space.

This fixes https://savannah.gnu.org/bugs/?60172.
Reported-by: Mike Frysinger <vapier@gentoo.org>

Bug existed since version 5.6, commit 1fdd23d3.

src/color.c

@@ -305,6 +305,7 @@ void precalc_multicolorinfo(void)
 
 	/* For each line, allocate cache space for the multiline-regex info. */
 	for (line = openfile->filetop; line != NULL; line = line->next)
+		if (!line->multidata)
 			line->multidata = nmalloc(openfile->syntax->nmultis * sizeof(short));
 
 	for (ink = openfile->syntax->color; ink != NULL; ink = ink->next) {