Fix SchemaContextUtil.findDataSchemaNodeForRelativeXPath()

On resolving relative paths for key_leaf1 and leaf2 leafrefs(with findDataSchemaNodeForRelativeXPath)  SchemaContextUtil.resolveRelativePath() returns incorrect paths.

module

module a {
  yang-version 1;
  namespace "a";
  prefix "a";

  grouping foo_grp {
    leaf key_leaf1 {
      type leafref {
        path "../../../../../../a:foo_cont/a:name";
      }
    }
  }

  container bar_cont {
    leaf leaf2 {
       type leafref {
        path "../../../../../../a:foo_cont/a:name";
      }
    }
  }
}

Incorrect paths:
for key_leaf1: [(a)foo_grp, (a)key_leaf1, (a)foo_cont, (a)name]

for leaf2: [(a)bar_cont, (a)leaf2, (a)foo_cont, (a)name]

On call findDataSchemaNodeForRelativeXPath with leaf2 fails with

java.lang.IllegalArgumentException: Path [(a)bar_cont, (a)leaf2, (a)foo_cont, (a)name] extends beyond terminal child

On call findDataSchemaNodeForRelativeXPath with key_leaf1 returns null.

Let's check whether the path goes behind grouping root node or root node of a different type. With this improvement, we will not lookup knowingly incorrect paths.