painting: properly detect a change in start/end matches

The inversion of the logic in commit a6605dce was incomplete -- oops.

This fixes https://savannah.gnu.org/bugs/?50306.

src/color.c

@@ -295,7 +295,7 @@ void color_update(void)
 void check_the_multis(filestruct *line)
 {
     const colortype *ink;
-    int astart, anend;
+    bool astart, anend;
     regmatch_t startmatch, endmatch;
 
     /* If there is no syntax or no multiline regex, there is nothing to do. */
@@ -309,8 +309,8 @@ void check_the_multis(filestruct *line)
 
 	alloc_multidata_if_needed(line);
 
-	astart = regexec(ink->start, line->data, 1, &startmatch, 0);
-	anend = regexec(ink->end, line->data, 1, &endmatch, 0);
+	astart = (regexec(ink->start, line->data, 1, &startmatch, 0) == 0);
+	anend = (regexec(ink->end, line->data, 1, &endmatch, 0) == 0);
 
 	/* Check whether the multidata still matches the current situation. */
 	if (line->multidata[ink->id] == CNONE ||