libdm: new name can't be empty

Do not allow passing '' names to kernel.

This test was missing also in kernel, so it has allowed
to create device with '' name.  This then confused dmsetup tool,
since such name is unexpected and unsupported. To remove
such name from table, user has to use -j -m to specify which device
should be removed.

This patch fixes the posibility to run this operation:

dmsetup rename existingdev ''

after this operation commands like  'dmsetup table' are failing.
This patch prohibits to use such name.

Author: Zdenek Kabelac

WHATS_NEW_DM

@@ -1,5 +1,6 @@
 Version 1.02.80 - 
 ==================================
+  Do not allow passing empty new name for dmsetup rename.
   Display any output returned by 'dmsetup message'.
   Add dm_task_get_message_response to libdevmapper.
 

libdm/libdm-common.c

@@ -698,6 +698,11 @@ int dm_task_set_newname(struct dm_task *dmt, const char *newname)
 		return 0;
 	}
 
+	if (!*newname) {
+		log_error("Non empty new name is required.");
+		return 0;
+	}
+
 	if (!check_multiple_mangled_string_allowed(newname, "new name", mangling_mode))
 		return_0;