Fix XPath binary expressions

Fix XPath binary expressions

AntlrXPathParser.parseRelationalExpr() is consuming acquiring
the operator twice, which leads to iterator state mismatch:

Caused by: java.util.NoSuchElementException
    at java.util.ArrayList$Itr.next (ArrayList.java:999)
    at org.opendaylight.yangtools.yang.xpath.impl.AntlrXPathParser.nextOperator (AntlrXPathParser.java:640)
    at org.opendaylight.yangtools.yang.xpath.impl.AntlrXPathParser.parseRelationalExpr (AntlrXPathParser.java:561)
    at org.opendaylight.yangtools.yang.xpath.impl.AntlrXPathParser.parseRelational (AntlrXPathParser.java:431)
    at org.opendaylight.yangtools.yang.xpath.impl.AntlrXPathParser.parseEquality (AntlrXPathParser.java:310)
    at org.opendaylight.yangtools.yang.xpath.impl.AntlrXPathParser.parseAnd (AntlrXPathParser.java:299)
    at org.opendaylight.yangtools.yang.xpath.impl.AntlrXPathParser.parseExpr (AntlrXPathParser.java:281)
    at org.opendaylight.yangtools.yang.xpath.impl.AntlrXPathParser.parseExpr (AntlrXPathParser.java:261)
    at org.opendaylight.yangtools.yang.xpath.impl.AntlrXPathParser$Qualified.parseExpression (AntlrXPathParser.java:151)

Fix this by properly reusing operator we have already acquired.

JIRA: YANGTOOLS-1045
Change-Id: I757412278eae2d92ff2422782d666cce5bca0720
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>