compiler: Allow string slices with start index == length.

    
    Avoid an off-by-one error when checking the start index of a string
    slice by allowing the start index to be the string length instead
    of the string length - 1.
    
    Fixes golang/go#11522.
    
    Reviewed-on: https://go-review.googlesource.com/13030

From-SVN: r227191

gcc/go/gofrontend/MERGE

-d6d59d5927c4ea0c02468ebc6a2df431fb64595a
+14ca4b6130b9a7132d132f418e9ea283b3a52c08
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -10341,7 +10341,10 @@ String_index_expression::do_check_types(Gogo*)
     {
       ival_valid = true;
       if (mpz_sgn(ival) < 0
-	  || (sval_valid && mpz_cmp_ui(ival, sval.length()) >= 0))
+	  || (sval_valid
+	      && (this->end_ == NULL
+		  ? mpz_cmp_ui(ival, sval.length()) >= 0
+		  : mpz_cmp_ui(ival, sval.length()) > 0)))
 	{
 	  error_at(this->start_->location(), "string index out of bounds");
 	  this->set_is_error();