Patch to error instead of crashing when parsing unmatched double quotes

Patch to error and fail instead of using all available memory
then crashing to detect the error condition of an unmatched
double quote before the end of a file.

I couldn't get it to show nice error messages, so this may not be
the ideal fix.

A test case for this situation has also been added.

src/comp/syntax/parse/lexer.rs

@@ -490,8 +490,15 @@ fn next_token_inner(rdr: reader) -> token::token {
         ret token::LIT_CHAR(c2);
       }
       '"' {
+        let n = rdr.get_chpos();
         rdr.bump();
         while rdr.curr() != '"' {
+            if rdr.is_eof() {
+                rdr.err(#fmt["unterminated double quote string: %s",
+                             rdr.get_str_from(n)]);
+                fail;
+            }
+
             let ch = rdr.curr();
             rdr.bump();
             alt ch {

src/test/compile-fail/unbalanced-doublequote.rs

@@ -0,0 +1,8 @@
+// -*- rust -*-
+
+// error-pattern: unterminated double quote string
+
+
+fn main() {
+    "
+}