Attempt to fix segfault in #156 (#157)

Author: quinnj

src/MySQL.jl

@@ -46,7 +46,7 @@ function clear!(conn)
 end
 
 function clear!(conn, result::API.MYSQL_RES)
-    if result.ptr != C_NULL
+    if conn.mysql.ptr != C_NULL && result.ptr != C_NULL
         while true
             if API.fetchrow(conn.mysql, result) == C_NULL
                 if API.moreresults(conn.mysql)

test/runtests.jl

@@ -268,3 +268,8 @@ ret = columntable(res)
 
 # multiple-queries not supported by mysql w/ prepared statements
 @test_throws MySQL.API.StmtError DBInterface.prepare(conn, "select * from Employee; select DeptNo, OfficeNo from Employee where OfficeNo IS NOT NULL")
+
+# 156
+res = DBInterface.execute(conn, "select * from Employee")
+DBInterface.close!(conn)
+ret = columntable(res)