Ensures that encountering an incomplete binary IVM causes clean failure.

src/main/java/com/amazon/ion/impl/IonCursorBinary.java

@@ -1113,6 +1113,9 @@ private void reset() {
      * point to the first byte after the IVM.
      */
     private void readIvm() {
+        if (limit < peekIndex + IVM_REMAINING_LENGTH) {
+            throw new IonException("Incomplete Ion version marker.");
+        }
         majorVersion = buffer[(int) (peekIndex++)];
         minorVersion = buffer[(int) (peekIndex++)];
         if ((buffer[(int) (peekIndex++)] & SINGLE_BYTE_MASK) != IVM_FINAL_BYTE) {

src/test/java/com/amazon/ion/impl/IonCursorBinaryTest.java

@@ -13,6 +13,7 @@
 import java.util.function.Consumer;
 
 import static com.amazon.ion.BitUtils.bytes;
+import static com.amazon.ion.IonCursor.Event.NEEDS_DATA;
 import static com.amazon.ion.IonCursor.Event.VALUE_READY;
 import static com.amazon.ion.IonCursor.Event.START_CONTAINER;
 import static com.amazon.ion.IonCursor.Event.START_SCALAR;
@@ -445,4 +446,25 @@ public void expectUseAfterCloseToHaveNoEffect(boolean constructFromBytes) {
         assertNull(cursor.getValueMarker().typeId);
         cursor.close();
     }
+
+    @ParameterizedTest(name = "constructFromBytes={0}")
+    @ValueSource(booleans = {true, false})
+    public void expectIncompleteIvmToFailCleanly(boolean constructFromBytes) {
+        IonCursorBinary cursor = initializeCursor(
+            constructFromBytes,
+            0xE0, 0x01, 0x00, 0xEA, // Complete IVM
+            0x20, // Int 0
+            0xE0, 0x01 // Incomplete IVM
+        );
+        assertEquals(START_SCALAR, cursor.nextValue());
+        if (constructFromBytes) {
+            // This is a fixed stream, so no more bytes will become available. An error must be raised when the
+            // incomplete IVM is encountered.
+            assertThrows(IonException.class, cursor::nextValue);
+        } else {
+            // This is a growing stream, so the cursor waits for more bytes.
+            assertEquals(NEEDS_DATA, cursor.nextValue());
+        }
+        cursor.close();
+    }
 }