painting: mark an unpaired start match as CWOULDBE

The lines that come after an unpaired start have to know about this.

This fixes https://savannah.gnu.org/bugs/?50293.

src/color.c

@@ -423,6 +423,12 @@ void precalc_multicolorinfo(void)
 	    /* Assume nothing applies until proven otherwise below. */
 	    line->multidata[ink->id] = CNONE;
 
+	    /* For an unpaired start match, mark all remaining lines. */
+	    if (line->prev && line->prev->multidata[ink->id] == CWOULDBE) {
+		line->multidata[ink->id] = CWOULDBE;
+		continue;
+	    }
+
 	    /* When the line contains a start match, look for an end, and if
 	     * found, mark all the lines that are affected. */
 	    while (regexec(ink->start, line->data + index, 1,
@@ -456,8 +462,10 @@ void precalc_multicolorinfo(void)
 		    tailline = tailline->next;
 		}
 
-		if (tailline == NULL)
+		if (tailline == NULL) {
+		    line->multidata[ink->id] = CWOULDBE;
 		    break;
+		}
 
 		/* We found it, we found it, la la la la la.  Mark all
 		 * the lines in between and the end properly. */

src/nano.h

@@ -288,6 +288,8 @@ typedef struct lintstruct {
 	/* Whole line engulfed by the regex, start < me, end > me. */
 #define CSTARTENDHERE	(1<<5)
 	/* Regex starts and ends within this line. */
+#define CWOULDBE	(1<<6)
+	/* An unpaired start match on or before this line. */
 #endif /* !DISABLE_COLOR */
 
 /* More structure types. */

src/winio.c

@@ -2506,8 +2506,10 @@ void edit_draw(filestruct *fileptr, const char *converted,
 		end_line = end_line->next;
 
 	    /* If there is no end, there is nothing to paint. */
-	    if (end_line == NULL)
+	    if (end_line == NULL) {
+		fileptr->multidata[varnish->id] = CWOULDBE;
 		goto tail_of_loop;
+	    }
 
 	    /* If the end is on a later line, paint whole line, and be done. */
 	    if (end_line != fileptr) {
@@ -2589,8 +2591,10 @@ void edit_draw(filestruct *fileptr, const char *converted,
 		    end_line = end_line->next;
 
 		/* If there is no end, we're done with this regex. */
-		if (end_line == NULL)
+		if (end_line == NULL) {
+		    fileptr->multidata[varnish->id] = CWOULDBE;
 		    break;
+		}
 
 		/* Paint the rest of the line. */
 		mvwaddnstr(edit, row, margin + start_col, thetext, -1);