fix possible SOE in multi-ile resolve

python/src/com/jetbrains/python/psi/resolve/PyResolveUtil.java

@@ -101,6 +101,7 @@ public class PyResolveUtil {
 
   @Nullable
   public static PsiElement getPrevNodeOf(PsiElement elt, PsiScopeProcessor proc) {
+    if (elt instanceof PsiFile) return null;  // no sense to get the previous node of a file
     if (proc instanceof PyClassScopeProcessor) {
       return getPrevNodeOf(elt, ((PyClassScopeProcessor)proc).getPossibleTargets());
     }

python/testData/resolve/multiFile/stackOverflowOnEmptyFile/StackOverflowOnEmptyFile.py

@@ -0,0 +1,4 @@
+from tornado.options import define
+
+define()
+# <ref>

python/testSrc/com/jetbrains/python/PyMultiFileResolveTest.java

@@ -134,6 +134,10 @@ public class PyMultiFileResolveTest extends PyLightFixtureTestCase {
     assertEquals("Context", ((PyClass) element).getName());
   }
 
+  public void testStackOverflowOnEmptyFile() throws Exception {
+    assertNull(doResolve());  // make sure we don't have a SOE here
+  }
+
   private PsiFile prepareFile() throws Exception {
     String testName = getTestName(true);
     String fileName = getTestName(false) + ".py";