patch 8.2.4439: accepting "iso8859" 'encoding' as "iso-8859-"

Problem:    Accepting "iso8859" 'encoding' as "iso-8859-".
Solution:   use "iso8859" as "iso-8859-1".
diff --git a/src/mbyte.c b/src/mbyte.c
index 2f4edd5..78230f6 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -318,6 +318,7 @@
 {
     {"ansi",		IDX_LATIN_1},
     {"iso-8859-1",	IDX_LATIN_1},
+    {"iso-8859",	IDX_LATIN_1},
     {"latin2",		IDX_ISO_2},
     {"latin3",		IDX_ISO_3},
     {"latin4",		IDX_ISO_4},
@@ -4523,7 +4524,7 @@
 	}
 
 	// "iso-8859n" -> "iso-8859-n"
-	if (STRNCMP(p, "iso-8859", 8) == 0 && p[8] != '-')
+	if (STRNCMP(p, "iso-8859", 8) == 0 && isdigit(p[8]))
 	{
 	    STRMOVE(p + 9, p + 8);
 	    p[8] = '-';