sha1_name: fix segfault caused by invalid index access

The code to see if user input "git show :path" makes sense tried to access
the index without properly checking the array bound.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_name.c

@@ -992,13 +992,15 @@ static void diagnose_invalid_index_path(int stage,
 	pos = cache_name_pos(filename, namelen);
 	if (pos < 0)
 		pos = -pos - 1;
-	ce = active_cache[pos];
-	if (ce_namelen(ce) == namelen &&
-	    !memcmp(ce->name, filename, namelen))
-		die("Path '%s' is in the index, but not at stage %d.\n"
-		    "Did you mean ':%d:%s'?",
-		    filename, stage,
-		    ce_stage(ce), filename);
+	if (pos < active_nr) {
+		ce = active_cache[pos];
+		if (ce_namelen(ce) == namelen &&
+		    !memcmp(ce->name, filename, namelen))
+			die("Path '%s' is in the index, but not at stage %d.\n"
+			    "Did you mean ':%d:%s'?",
+			    filename, stage,
+			    ce_stage(ce), filename);
+	}
 
 	/* Confusion between relative and absolute filenames? */
 	fullnamelen = namelen + strlen(prefix);
@@ -1008,13 +1010,15 @@ static void diagnose_invalid_index_path(int stage,
 	pos = cache_name_pos(fullname, fullnamelen);
 	if (pos < 0)
 		pos = -pos - 1;
-	ce = active_cache[pos];
-	if (ce_namelen(ce) == fullnamelen &&
-	    !memcmp(ce->name, fullname, fullnamelen))
-		die("Path '%s' is in the index, but not '%s'.\n"
-		    "Did you mean ':%d:%s'?",
-		    fullname, filename,
-		    ce_stage(ce), fullname);
+	if (pos < active_nr) {
+		ce = active_cache[pos];
+		if (ce_namelen(ce) == fullnamelen &&
+		    !memcmp(ce->name, fullname, fullnamelen))
+			die("Path '%s' is in the index, but not '%s'.\n"
+			    "Did you mean ':%d:%s'?",
+			    fullname, filename,
+			    ce_stage(ce), fullname);
+	}
 
 	if (!lstat(filename, &st))
 		die("Path '%s' exists on disk, but not in the index.", filename);