Don't leave is_block in bad state after bad redirection

Fixes #1728.

exec.cpp

@@ -516,9 +516,6 @@ static void internal_exec_helper(parser_t &parser,
     std::vector<int> opened_fds;
     bool transmorgrified = io_transmogrify(ios, &morphed_chain, &opened_fds);
 
-    int is_block_old=is_block;
-    is_block=1;
-
     /*
       Did the transmogrification fail - if so, set error status and return
     */
@@ -528,6 +525,9 @@ static void internal_exec_helper(parser_t &parser,
         return;
     }
 
+    int is_block_old=is_block;
+    is_block=1;
+
     signal_unblock();
 
     if (node_offset == NODE_OFFSET_INVALID)

tests/gen_output.fish

@@ -1,4 +1,4 @@
-#!/usr/bin/fish
+#!/usr/bin/env fish
 
 #
 # Generate output for a test script

tests/status.err

@@ -0,0 +1,2 @@
+fish: An error occurred while redirecting file '/'
+open: Is a directory

tests/status.in

@@ -0,0 +1,9 @@
+# vim: set filetype=fish:
+
+# Issue #1728
+# Bad file redirection on a block causes `status --is-block` to return 0 forever.
+begin; end >/ # / is a directory, it can't be opened for writing
+status -b
+and echo 'block'
+
+true

tests/status.status

@@ -0,0 +1 @@
+0

tests/top.out

@@ -2,6 +2,7 @@ Testing high level script functionality
 File expansion.in tested ok
 File printf.in tested ok
 File read.in tested ok
+File status.in tested ok
 File test1.in tested ok
 File test2.in tested ok
 File test3.in tested ok