Fixed test case for bug #13105873 "valgrind warning:possible

crash in foreign key handling on subsequent create table if
not exists".

Do not leave current database unassigned after the end of
the test case.

mysql-test/r/sp-bugs.result

@@ -128,4 +128,5 @@ CALL p1();
 Warnings:
 Note	1050	Table 't2' already exists
 DROP DATABASE testdb;
+USE test;
 End of 5.1 tests

mysql-test/t/sp-bugs.test

@@ -161,5 +161,6 @@ CALL p1();
 --echo # below stmt should not return valgrind warnings
 CALL p1(); 
 DROP DATABASE testdb;
+USE test;
 
 --echo End of 5.1 tests