subtract 'a' from indices for marks

Given:

static int marks['z' - 'a'];
marks[c];

In this case c is the character read and index is out of bounds. We
need marks[c - 'a']

While playing around with optimization settings gcc caught caught this.

Also fix one check for c, change from isalpha to islower.

-emg

From 2cc36818283e9068576c1042690c016a81b709a3 Mon Sep 17 00:00:00 2001
From: Evan Gates <evan.gates@gmail.com>
Date: Fri, 15 Jul 2016 09:52:39 -0700
Subject: [PATCH] fix marks indexing

ed.c

@@ -478,9 +478,9 @@ linenum(int *line)
 		break;
 	case '\'':
 		skipblank();
-		if (!isalpha(c = input()))
+		if (!islower(c = input()))
 			error("invalid mark character");
-		if (!(ln = marks[c]))
+		if (!(ln = marks[c - 'a']))
 			error("invalid address");
 		break;
 	case '$':
@@ -1191,7 +1191,7 @@ repeat:
 			error("invalid mark character");
 		chkprint(1);
 		deflines(curln, curln);
-		marks[c] = line1;
+		marks[c - 'a'] = line1;
 		break;
 	case 'P':
 		if (nlines > 0)