vietnamese/vnconvert: Replace gets(3) with gets_s(3)

The long deprecated and unsafe gets(3) function has been removed since
FreeBSD 1300043.

Thus replace the occurences of that function in the code with gets_s(3)
which is a safer alternative.

* Bump PORTREVISION due to changes in the code/package.

PR:		238687
Reported by:	emaste
MFH:		2019Q3

vietnamese/vnconvert/Makefile

@@ -3,6 +3,7 @@
 
 PORTNAME=	vnconvert
 PORTVERSION=	1.0
+PORTREVISION=	1
 CATEGORIES=	vietnamese
 MASTER_SITES=	LOCAL/obrien
 DISTNAME=	convert

vietnamese/vnconvert/files/patch-vnconvert.c

@@ -0,0 +1,45 @@
+--- vnconvert.c.orig	1994-05-20 20:36:17 UTC
++++ vnconvert.c
+@@ -91,7 +91,7 @@ main()
+ 		printf("\t6 - VN Thin-Art (Mac)\n\n");
+ 
+ 		printf("Input-file name (or Enter to quit): ");
+-		gets(bufline);
++		gets_s(bufline, 255);
+ 		if (strlen(bufline) == 0)
+ 			break;
+ 		sscanf(bufline, "%s", input_file);
+@@ -99,21 +99,21 @@ main()
+ 		printf("currently in the format number: ");
+ 		*bufline = '\0'; 
+ 		while (strlen(bufline)==0) {
+-			gets(bufline);
++			gets_s(bufline, 255);
+ 			sscanf(bufline, "%d", &from);
+ 		}
+ 
+ 		printf("\nOutput-file name: ");
+ 		*bufline = '\0';
+                 while (strlen(bufline)==0) {
+-                        gets(bufline);
++                        gets_s(bufline, 255);
+                         sscanf(bufline, "%s", output_file); 
+                 }
+ 
+ 		printf("to the format number: ");
+ 		*bufline = '\0';
+                 while (strlen(bufline)==0) {
+-                        gets(bufline);
++                        gets_s(bufline, 255);
+                         sscanf(bufline, "%d", &to);
+                 }
+ 
+@@ -152,7 +152,7 @@ main()
+ 			}
+ 			else {
+ 				fprintf(stderr, "\"%s\" already exists. Overwrite it (y/n)? ", output_file);
+-				gets(buf);
++				gets_s(buf, BUFSIZ);
+ 				if (toupper(buf[0]) != 'Y')  
+ 					icontinue = 1;
+ 			}