Fix with clang 15: invalid pointer to integer conversion.

Index: mgp.c
--- mgp.c.orig
+++ mgp.c
@@ -506,16 +506,16 @@ genhtml(start_page)
 	char *childdebug;
 	char *convdb[][3] = {{ "jpg", "cjpeg", "djpeg" },
 				{ "png", "pnmtopng", "pngtopnm" },
-				{ NULL, NULL, NULL }};
+				{ "", "", "" }};
 	int inum = 0;
 
 	/* check image type */
 	if (htmlimage) {
-		for (inum = 0; *convdb[inum] != NULL; inum++) {
+		for (inum = 0; **convdb[inum] != '\0'; inum++) {
 			if (strcmp(*convdb[inum], htmlimage) == 0)
 				break;
 		}
-		if (*convdb[inum] == NULL) {
+		if (**convdb[inum] == '\0') {
 			fprintf(stderr, "unknwon image type %s.\n", htmlimage);
 			/* print out valid image types */
 			fprintf(stderr, "Valid image types: ");