Show no preview for an entry with no address

Ex mode starts on the last line, so the preview reported the end of the file as
the tag, while every other path refuses such an entry.

bin/tagpreview.sh

@@ -30,6 +30,12 @@ else
   exit 1
 fi
 
+# Ex mode starts on the last line, so an entry with no address would report the
+# end of the file as the tag. Every other path refuses such an entry
+if [ -z "${EXCMD}" ]; then
+  exit 1
+fi
+
 # The address comes from a tags file, which can be untrusted, and a preview runs
 # as soon as an entry is highlighted. Sandboxed as a builtin tag jump is, so that
 # it cannot run a shell command or touch a file. ':sandbox' on its own only covers