Fix ArrayIndexOutOfBoundsException in conv()

scala> sql(s"SELECT CONV('${Long.MinValue}', 10, -2)").show(false)
java.lang.ArrayIndexOutOfBoundsException: -1
  at org.apache.spark.sql.catalyst.util.NumberConverter$.convert(NumberConverter.scala:183)
  at org.apache.spark.sql.catalyst.expressions.Conv.nullSafeEval(mathExpressions.scala:463)
  at org.apache.spark.sql.catalyst.expressions.TernaryExpression.eval(Expression.scala:821)
  at org.apache.spark.sql.catalyst.expressions.ToPrettyString.eval(ToPrettyString.scala:57)
  at org.apache.spark.sql.catalyst.optimizer.ConstantFolding$.org$apache$spark$sql$catalyst$optimizer$ConstantFolding$$constantFolding(expressions.scala:81)
  at org.apache.spark.sql.catalyst.optimizer.ConstantFolding$.$anonfun$constantFolding$4(expressions.scala:91)