[dbsystel] Fix segfault if fd unavailable

The `get_java_var_long` function returns 0 in several failure modes,
e.g. if a file descriptor is unavailable. [1]

However, one of the call sites is missing the result check, which causes
a JVM segfault if the return value is 0. The segfault occurs on
dereferencing the pointer: [2]

```c
eis->eventflags[SPE_DATA_AVAILABLE]
```

Add a result value check, throwing a proper IOException if it is 0.

See also similar issue NeuronRobotics#59. [3]

Fixes NeuronRobotics#112 [4], NeuronRobotics#136 [5] and NeuronRobotics#242 [6].

[1]: https://github.com/NeuronRobotics/nrjavaserial/blob/0df8b60485a56d7698b71183237b5615d02a8194/src/main/c/src/SerialImp.c#L5137-L5142
[2]: https://github.com/NeuronRobotics/nrjavaserial/blob/0df8b60485a56d7698b71183237b5615d02a8194/src/main/c/src/SerialImp.c#L3085
[3]: NeuronRobotics#59
[4]: NeuronRobotics#112
[5]: NeuronRobotics#136
[6]: NeuronRobotics#242

Reported-by: Alex Vasiliev <@alex-vas>
Reported-by: Łukasz Dywicki <luke@code-house.org>
Reported-by: Jose Pacelli <josepacelli@gmail.com>
Reported-by: Frank Hartwig <frank.fr.hartwig@deutschebahn.com>

src/main/c/src/SerialImp.c

@@ -3081,6 +3081,11 @@ int read_byte_array( JNIEnv *env,
 	struct event_info_struct *eis = ( struct event_info_struct * )
 		get_java_var_long( env, *jobj,"eis","J" );
 
+	if (eis == NULL) {
+		throw_java_exception(env, IO_EXCEPTION, "read_byte_array",
+			"Unable to read byte array");
+		return -1;
+	}
 	report_time_start();
 	flag = eis->eventflags[SPE_DATA_AVAILABLE];
 	eis->eventflags[SPE_DATA_AVAILABLE] = 0;