check for negative length in CigarElement ctor

src/main/java/htsjdk/samtools/CigarElement.java

@@ -36,6 +36,7 @@
     private final CigarOperator operator;
 
     public CigarElement(final int length, final CigarOperator operator) {
+        if (length < 0) throw new IllegalArgumentException(String.format("Cigar element being constructed with negative length: %d and operation: %s" , length, operator.name()));
         this.length = length;
         this.operator = operator;
     }

src/test/java/htsjdk/samtools/util/CigarElementUnitTest.java

@@ -0,0 +1,14 @@
+package htsjdk.samtools.util;
+
+
+import htsjdk.samtools.CigarElement;
+import htsjdk.samtools.CigarOperator;
+import org.testng.annotations.Test;
+
+public class CigarElementUnitTest {
+
+    @Test(expectedExceptions = IllegalArgumentException.class)
+    public void testNegativeLengthCheck(){
+        final CigarElement element = new CigarElement(-1, CigarOperator.M);
+    }
+}