fix #1177 (LB(k) stops at index 0) @antlr/antlr-targets might care about this

runtime/Java/src/org/antlr/v4/runtime/CommonTokenStream.java

@@ -77,7 +77,7 @@ public class CommonTokenStream extends BufferedTokenStream {
         int i = p;
         int n = 1;
         // find k good tokens looking backwards
-        while ( n<=k ) {
+        while ( n<=k && i>0 ) {
             // skip off-channel tokens
             i = previousTokenOnChannel(i - 1, channel);
             n++;