fix: validate non-numeric subnet prefixes

Author: dkropachev

core/src/main/java/com/datastax/oss/driver/internal/core/addresstranslation/Subnet.java

@@ -55,7 +55,13 @@ static Subnet parse(String subnetCIDR) throws UnknownHostException {
     if (isIPv4(subnet) && isIPv6) {
       subnet = toIPv6(subnet);
     }
-    int prefixLength = Integer.parseInt(parts.get(1));
+    int prefixLength;
+    try {
+      prefixLength = Integer.parseInt(parts.get(1));
+    } catch (NumberFormatException e) {
+      throw new IllegalArgumentException(
+          String.format("Prefix length %s must be a number", parts.get(1)), e);
+    }
     validatePrefixLength(subnet, prefixLength);
 
     byte[] networkMask = toNetworkMask(subnet, prefixLength);

core/src/test/java/com/datastax/oss/driver/internal/core/addresstranslation/SubnetTest.java

@@ -85,6 +85,13 @@ public void should_fail_on_invalid_prefix_length() {
         .withMessage("Prefix length 33 must be within [0; 32]");
   }
 
+  @Test
+  public void should_fail_on_non_numeric_prefix_length() {
+    assertThatIllegalArgumentException()
+        .isThrownBy(() -> Subnet.parse("100.64.0.0/foo"))
+        .withMessage("Prefix length foo must be a number");
+  }
+
   @Test
   public void should_fail_on_not_prefix_block_subnet_ipv4() {
     assertThatIllegalArgumentException()