[NaCl SDK] nacl_io: Fix bug in Path::Range introduced in recent change.

Path("foo/bar") has 2 components "foo" and "bar". It is legal when calling
Path::Range to access an invalid component (e.g. Path("foo/bar").Range(2, 2).
In this case, the previous STL-based implementation would return "", but my
non-STL implementation was incorrectly returning "foo/bar".

BUG=none
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/1018393007

Cr-Commit-Position: refs/heads/master@{#322035}

native_client_sdk/src/libraries/nacl_io/path.cc

@@ -170,7 +170,7 @@ std::string Path::Range(size_t start, size_t end) const {
     }
   }
 
-  if (pstart > pend)
+  if (slashes < start || pstart > pend)
     return std::string();
 
   return std::string(pstart, pend - pstart);

native_client_sdk/src/tests/nacl_io_test/path_test.cc

@@ -70,6 +70,7 @@ TEST(PathTest, OnePart_Absolute) {
   EXPECT_EQ("/", p.Range(0, 1));
   EXPECT_EQ("foo", p.Range(1, 2));
   EXPECT_EQ("/foo", p.Range(0, 2));
+  EXPECT_EQ("", p.Range(2, 2));
   EXPECT_EQ("/", p.Parent().Join());
 }
 
@@ -85,6 +86,7 @@ TEST(PathTest, TwoPart_Relative) {
   EXPECT_EQ("foo", p.Range(0, 1));
   EXPECT_EQ("bar", p.Range(1, 2));
   EXPECT_EQ("foo/bar", p.Range(0, 2));
+  EXPECT_EQ("", p.Range(2, 2));
   EXPECT_EQ("foo", p.Parent().Join());
 }
 
@@ -185,6 +187,9 @@ TEST(PathTest, Range_Relative) {
   EXPECT_EQ("relative/path", p.Range(1, 3));
 
   EXPECT_EQ("path", p.Range(2, 3));
+
+  EXPECT_EQ("", p.Range(2, 100));
+  EXPECT_EQ("", p.Range(42, 67));
 }
 
 TEST(PathTest, Range_Absolute) {
@@ -203,6 +208,9 @@ TEST(PathTest, Range_Absolute) {
   EXPECT_EQ("absolute/path", p.Range(2, 4));
 
   EXPECT_EQ("path", p.Range(3, 4));
+
+  EXPECT_EQ("", p.Range(2, 100));
+  EXPECT_EQ("", p.Range(42, 67));
 }
 
 TEST(PathTest, Assign) {