Fixed segfault caused by image removal in thumbnail mode

Makefile

@@ -1,4 +1,4 @@
-VERSION = git-20140901
+VERSION = git-20140911
 
 PREFIX    = /usr/local
 MANPREFIX = $(PREFIX)/share/man

main.c

@@ -187,7 +187,7 @@ void remove_file(int n, bool manual)
 	filecnt--;
 	if (n < tns.cnt)
 		tns.cnt--;
-	if (mode == MODE_THUMB && fileidx >= tns.cnt)
+	if (mode == MODE_THUMB && tns.cnt > 0 && fileidx >= tns.cnt)
 		fileidx = tns.cnt - 1;
 	if (n < alternate)
 		alternate--;