Validate that there is indeed a field to set the encoding to, in the IsoMessage. Update version to 1.5.1

git-svn-id: https://j8583.svn.sourceforge.net/svnroot/j8583/j8583@72 db60a039-f72e-0410-b1d2-baa000846441

src/main/java/com/solab/iso8583/IsoMessage.java

@@ -127,7 +127,9 @@ public class IsoMessage {
     	if (index < 2 || index > 128) {
     		throw new IndexOutOfBoundsException("Field index must be between 2 and 128");
     	}
-    	field.setCharacterEncoding(encoding);
+    	if (field != null) {
+        	field.setCharacterEncoding(encoding);
+    	}
     	fields[index] = field;
     }