[gfortran,patch] Fix zero-length substrings

[FX Coudert]

> Ok, with the change to MAX_EXPR if you like.

Probably one of the quickest review in gfortran history! Thanks Tobias.

Here is what I commited:

Index: trans-expr.c
===================================================================
--- trans-expr.c        (revision 114461)
+++ trans-expr.c        (working copy)
@@ -275,6 +275,8 @@
                      build_int_cst (gfc_charlen_type_node, 1),
                      start.expr);
    tmp = fold_build2 (PLUS_EXPR, gfc_charlen_type_node, end.expr, tmp);
+  tmp = fold_build2 (MAX_EXPR, gfc_charlen_type_node, tmp,
+                    build_int_cst (gfc_charlen_type_node, 0));
    se->string_length = tmp;
}
@@ -2196,6 +2198,7 @@
    tree tmp;
    tree dsc;
    tree ssc;
+  tree cond;
    /* Deal with single character specially.  */
    dsc = gfc_to_single_character (dlen, dest);
@@ -2206,12 +2209,16 @@
        return;
      }
+  cond = fold_build2 (GT_EXPR, boolean_type_node, dlen,
+                     build_int_cst (gfc_charlen_type_node, 0));
+
    tmp = NULL_TREE;
    tmp = gfc_chainon_list (tmp, dlen);
    tmp = gfc_chainon_list (tmp, dest);
    tmp = gfc_chainon_list (tmp, slen);
    tmp = gfc_chainon_list (tmp, src);
    tmp = build_function_call_expr (gfor_fndecl_copy_string, tmp);
+  tmp = fold_build3 (COND_EXPR, void_type_node, cond, tmp,  
build_empty_stmt ());
    gfc_add_expr_to_block (block, tmp);
}