Don't allow TextFile to be initialised with null inputs

Issue 3703

Change-Id: I5b58a915dee69bb88abc63771114562d96d4e5b5
Reviewed-on: http://gerrit.dmdirc.com/916
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>
Reviewed-by: Gregory Holmes <greg@dmdirc.com>

src/com/dmdirc/util/TextFile.java

      * @since 0.6.3m1
      */
     public TextFile(final File file, final Charset charset) {
+        if (file == null) {
+            throw new NullPointerException("file");
+        }
+
         this.file = file;
         this.charset = charset;
     }
      * @since 0.6.3m1
      */
     public TextFile(final InputStream is, final Charset charset) {
+        if (is == null) {
+            throw new NullPointerException("is");
+        }
+
         this.is = is;
         this.charset = charset;
     }