Fix incomplete source exceptions in repl

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/parser/PythonErrorStrategy.java

@@ -81,7 +81,7 @@ private static String getTokeLineText(Parser recognizer, Token token) {
         int index = token.getTokenIndex();
         // search for line start
         int tokenIndex = index;
-        int start = -1;
+        int start = 0;
         while (tokenIndex >= 0) {
             Token t = tokenStream.get(tokenIndex);
             if (t.getText().equals("\n")) {