Fix missing bounds checks for JsonTreeReader.getPath() (#2001)

There are situations where the stack of JsonTreeReader contains a JsonArray
or JsonObject without a subsequent Iterator, for example after calling peek()
or nextName().
When JsonTreeReader.getPath() is called afterwards it therefore must not
assume that a JsonArray or JsonObject is always followed by an Iterator.

The only reason why this never caused an ArrayIndexOutOfBoundsException in
the past is because the stack has an even default size (32) so it would just
have read the next `null`.
However, if the stack had for example the default size 31, a user created a
JsonTreeReader for 16 JSON arrays nested inside each other, then called 15
times beginArray(), followed by peek() and getPath() the exception would
occur.

gson/src/main/java/com/google/gson/internal/bind/JsonTreeReader.java

@@ -308,11 +308,11 @@ private void push(Object newTop) {
     StringBuilder result = new StringBuilder().append('$');
     for (int i = 0; i < stackSize; i++) {
       if (stack[i] instanceof JsonArray) {
-        if (stack[++i] instanceof Iterator) {
+        if (++i < stackSize && stack[i] instanceof Iterator) {
           result.append('[').append(pathIndices[i]).append(']');
         }
       } else if (stack[i] instanceof JsonObject) {
-        if (stack[++i] instanceof Iterator) {
+        if (++i < stackSize && stack[i] instanceof Iterator) {
           result.append('.');
           if (pathNames[i] != null) {
             result.append(pathNames[i]);