compiler: Check for invalid UTF8 in Go comments.

Fixes golang/go#11527.

Change-Id: I94fd08ae67393c3422858bc6d1d1311d590c842f
Reviewed-on: https://go-review.googlesource.com/13905
Reviewed-by: Ian Lance Taylor <iant@golang.org>

go/lex.cc

@@ -1689,6 +1689,16 @@ Lex::skip_cpp_comment()
       && memcmp(p, "line ", 5) == 0)
     {
       p += 5;
+
+      // Before finding FILE:LINENO, make sure line has valid characters.
+      const char* pcheck = p;
+      while (pcheck < pend)
+        {
+          unsigned int c;
+          bool issued_error;
+          pcheck = this->advance_one_utf8_char(pcheck, &c, &issued_error);
+        }
+
       while (p < pend && *p == ' ')
 	++p;
       const char* pcolon = static_cast<const char*>(memchr(p, ':', pend - p));