Fixed some craches in axutil_hash.

 * Moved calls of the function 'axutil_env_increment_ref' to the
   proper places.
 * Added some checks for allocated memory (compare with NULL).
 * Compare env with ht->env in 'axutil_hash_set_env' function to prevent
   freeing of the env element before assigning if both pointers
   point to the same memory location.
Fixed some more segfaults.

 * Compare out_transport_info->ops with NULL to prevent segfault
   in AXIS2_OUT_TRANSPORT_INFO_FREE macro.
 * Compare svc_ctx with NULL before calling axis2_op_ctx_set_parent
   in function 'axis2_op_ctx_create'.
 * Added checks of in arguments of functions for axis2_svc_grp_ctx_t
   and axis2_op_t structures.
 * Compare op with NULL in axis2_engine_ functions.