src/exec: fix assertion on failed exec redirection

Minimal reproducer: `fish -c "exec cat<x"`

src/exec.cpp

@@ -970,7 +970,11 @@ bool exec_job(parser_t &parser, shared_ptr<job_t> j) {
 
     if (j->processes.front()->type == INTERNAL_EXEC) {
         internal_exec(parser.vars(), j.get(), all_ios);
-        DIE("this should be unreachable");
+        // internal_exec only returns if it failed to set up redirections.
+        // In case of an successful exec, this code is not reached.
+        bool status = j->get_flag(job_flag_t::NEGATE) ? 0 : 1;
+        proc_set_last_status(status);
+        return false;
     }
 
     // This loop loops over every process_t in the job, starting it as appropriate. This turns out

tests/test_exec_fail.err

@@ -0,0 +1,4 @@
+<W> fish: An error occurred while redirecting file 'nosuchfile'
+open: No such file or directory
+<W> fish: An error occurred while redirecting file 'nosuchfile'
+open: No such file or directory

tests/test_exec_fail.in

@@ -0,0 +1,6 @@
+exec cat < nosuchfile
+echo "failed: $status"
+not exec cat < nosuchfile
+echo "neg failed: $status"
+exec cat < /dev/null
+echo "not reached"

tests/test_exec_fail.out

@@ -0,0 +1,2 @@
+failed: 1
+neg failed: 0

tests/test_exec_fail.status

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