param_get: add null-pointer check

If param_find() returned PARAM_INVALID, and this was directly passed to
param_get(), param_get_value_ptr() returned null and we read garbage data
(or segfaulted on systems with virtual memory).
On px4fmu-v2, this happened for the param ATT_VIBE_THRESH in sensors.
Because of the recently added parameter scoping, this param got pruned, as
it's defined in attitude_estimator_q.

credits for finding this go to Jeyong Shin (jeyong).

src/modules/systemlib/param/param.c

@@ -531,7 +531,7 @@ param_get(param_t param, void *val)
 
 	const void *v = param_get_value_ptr(param);
 
-	if (val != NULL) {
+	if (val && v) {
 		memcpy(val, v, param_size(param));
 		result = 0;
 	}

src/modules/systemlib/param/param_shmem.c

@@ -548,7 +548,7 @@ param_get(param_t param, void *val)
 
 	const void *v = param_get_value_ptr(param);
 
-	if (val != NULL) {
+	if (val && v) {
 		memcpy(val, v, param_size(param));
 		result = 0;
 	}