Fix to bugs #665, and #664. Potentially to #649.

http://bugs.skysql.com/show_bug.cgi?id=665
http://bugs.skysql.com/show_bug.cgi?id=664
http://bugs.skysql.com/show_bug.cgi?id=649

dcb.c:dcb_final_free: (665):set dcb->session->client pointer to NULL so that it won't be read anymore and other threads won't try to close it.
	dcb_final_free:(664):don't free dcb->data, it is either freed in session_alloc if session creation fails or in session_free only.
session.c:if session creation fails, free dcb->data and remove links between client DCB and session.
mysql_backend.c:(665):gw_backend_close:check that session->client isn't NULL and that client DCB's state is still polling before calling dcb_close for it.
mysql_client.c:gw_mysql_do_authentication:if anything fails, and session_alloc won't be called, free dcb->data.
mysql_common.c:gw_send_authentication_to_backend:if session is already closing then return with error.

utils/skygw_debug.h

@@ -124,7 +124,8 @@ typedef enum skygw_chk_t {
     CHK_NUM_BACKEND,
     CHK_NUM_BACKEND_REF,
     CHK_NUM_PREP_STMT,
-    CHK_NUM_PINFO
+    CHK_NUM_PINFO,
+    CHK_NUM_MYSQLSES
 } skygw_chk_t;
 
 # define STRBOOL(b) ((b) ? "true" : "false")
@@ -542,6 +543,11 @@ typedef enum skygw_chk_t {
         "Parsing info struct has invalid check fields");                \
 }
 
+#define CHK_MYSQL_SESSION(s) {						\
+	ss_info_dassert((s)->myses_chk_top == CHK_NUM_MYSQLSES &&	\
+	(s)->myses_chk_tail == CHK_NUM_MYSQLSES,			\
+	"MYSQL session struct has invalid check fields");		\
+}
 
 
 #if defined(FAKE_CODE)