add unit test for primefield infinite loop

tests/Unit/Math/PrimeFieldTest.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace phpseclib3\Tests\Unit\Math;
+
+use phpseclib3\Math\BigInteger;
+use phpseclib3\Math\PrimeField;
+use phpseclib3\Tests\PhpseclibTestCase;
+
+class PrimeFieldTest extends PhpseclibTestCase
+{
+    public function testPrimeFieldWithCompositeNumbers()
+    {
+        $this->expectException('UnexpectedValueException');
+
+        $a = new BigInteger('65', 10);
+        $p = new BigInteger('126', 10); // 126 isn't a prime
+
+        $num = new PrimeField($p);
+        $num2 = $num->newInteger($a);
+
+        echo $num2->squareRoot();
+    }
+}