support hippie completion at the file start (EA-52100)

java/java-tests/testSrc/com/intellij/codeInsight/completion/HippieCompletionTest.groovy

@@ -128,6 +128,19 @@ class Xoo {
 
   }
 
+  public void "test file start"() {
+    myFixture.configureByText "a.java", '''<caret>
+class Xoo {
+}
+'''
+    complete()
+    myFixture.checkResult '''class<caret>
+class Xoo {
+}
+'''
+
+  }
+
   private void complete() {
     myFixture.performEditorAction(IdeActions.ACTION_HIPPIE_COMPLETION)
   }

platform/lang-impl/src/com/intellij/codeInsight/completion/actions/HippieWordCompletionHandler.java

@@ -305,7 +305,7 @@ public class HippieWordCompletionHandler implements CodeInsightActionHandler {
 
     final CompletionData data = new CompletionData();
     
-    processWords(editor, offset - 1, new TokenProcessor() {
+    processWords(editor, Math.max(offset - 1, 0), new TokenProcessor() {
       @Override
       public boolean processToken(int start, int end) {
         if (start > offset) {