HTMLPreloadScanner should only use valid <base> urls

LayoutTests/fast/parser/badurl-head-preloader-crash.html

Index: LayoutTests/fast/parser/badurl-head-preloader-crash.html
diff --git a/LayoutTests/fast/parser/badurl-head-preloader-crash.html b/LayoutTests/fast/parser/badurl-head-preloader-crash.html
new file mode 100644
index 0000000000000000000000000000000000000000..e23986c8d8d30c826570b7b36b0dc1ade295be9a
--- /dev/null
+++ b/LayoutTests/fast/parser/badurl-head-preloader-crash.html
@@ -0,0 +1,6 @@
+<base href="gopher:��&#279%�:0"></base>
+<script src=":"></script>

[Yoav Weiss, 2015/03/31 11:04:43]

Can you add another test where we see that the invalid base URL is not taken
into account?

[kouhei (in TOK), 2015/03/31 11:15:38]

Acknowledged.

[kouhei (in TOK), 2015/04/01 03:49:05]

Done.

+<script>
+if (window.testRunner) testRunner.dumpAsText();
+</script>
+<p>PASS: if scanning this document with preloader doesn't crash in debug builds</p>