Bugfix: check context manager deinit callback for NULL before calling it (#2222)

The deinit_cb should be allowed to be NULL.

JerryScript-DCO-1.0-Signed-off-by: Martijn The martijn.the@intel.com

docs/02.API-REFERENCE.md

@@ -122,8 +122,9 @@ typedef uint32_t jerry_value_t;
 **Summary**
 
 Structure that defines how a context data item will be initialized and deinitialized. JerryScript zeroes out the memory
-for the item by default, and if the `init_cb` field is not NULL, it will be called with the pointer to the memory  as
-an additional custom initializer.
+for the item by default, and if the `init_cb` field is not NULL, it will be called with the pointer to the memory as
+an additional custom initializer. The `deinit_cb` (if non-`NULL`) is called during a call to `jerry_cleanup()` to run
+any custom deinitialization.
 
 **Prototype**
 
@@ -131,7 +132,7 @@ an additional custom initializer.
 typedef struct
 {
   void (*init_cb) (void *); /**< callback responsible for initializing a context item, or NULL */
-  void (*deinit_cb) (void *); /**< callback responsible for deinitializing a context item */
+  void (*deinit_cb) (void *); /**< callback responsible for deinitializing a context item, or NULL */
   size_t bytes_needed; /**< number of bytes to allocate for this manager */
 } jerry_context_data_manager_t;
 ```