symbolic exports: fix ReleaseStringUTFChars call on null pointer

In case that no export_name is passed to one of the PM_Export..., PS_Export.... functions,
ReleaseStringUTFChars() would be called even though there was no previous call to
GetStringUTFChars(), leading to a SEGFAULT.

This is for robustness, PRISM always passes an export_name string.

prism/src/mtbdd/PM_ExportLabels.cc

@@ -127,7 +127,7 @@ jstring fn		// filename
 	
 	// close file, etc.
 	if (export_file) fclose(export_file);
-	env->ReleaseStringUTFChars(na, export_name);
+	if (na) env->ReleaseStringUTFChars(na, export_name);
 	
 	return 0;
 }

prism/src/mtbdd/PM_ExportMatrix.cc

@@ -86,7 +86,7 @@ jstring fn		// filename
 	
 	// close file, etc.
 	if (export_file) fclose(export_file);
-	env->ReleaseStringUTFChars(na, export_name);
+	if (na) env->ReleaseStringUTFChars(na, export_name);
 	
 	return 0;
 }

prism/src/mtbdd/PM_ExportVector.cc

@@ -76,7 +76,7 @@ jstring fn		// filename
 	
 	// close file, etc.
 	if (export_file) fclose(export_file);
-	env->ReleaseStringUTFChars(na, export_name);
+	if (na) env->ReleaseStringUTFChars(na, export_name);
 	
 	return 0;
 }

prism/src/sparse/PS_ExportMatrix.cc

@@ -162,7 +162,7 @@ jstring fn		// filename
 	
 	// close file, etc.
 	if (export_file) fclose(export_file);
-	env->ReleaseStringUTFChars(na, export_name);
+	if (na) env->ReleaseStringUTFChars(na, export_name);
 	
 	// catch exceptions: return (undocumented) error code for memout
 	} catch (std::bad_alloc e) {

prism/src/sparse/PS_ExportSubMDP.cc

@@ -120,7 +120,7 @@ jstring fn		// filename
 	
 	// close file, etc.
 	if (export_file) fclose(export_file);
-	env->ReleaseStringUTFChars(na, export_name);
+	if (na) env->ReleaseStringUTFChars(na, export_name);
 	
 	// catch exceptions: return (undocumented) error code for memout
 	} catch (std::bad_alloc e) {