Fixup gdb.base/tracefork-zombie.exp

Fix ERROR:
...
PASS: gdb.base/tracefork-zombie.exp: attach
ERROR: tcl error sourcing gdb/testsuite/gdb.base/tracefork-zombie.exp.
ERROR: tcl error code POSIX ESRCH {no such process}
ERROR: error reading "file12": no such process
    while executing
"read $statusfi"
    ("foreach" body line 5)
    invoked from within
...

---
 gdb/testsuite/gdb.base/tracefork-zombie.exp | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/gdb/testsuite/gdb.base/tracefork-zombie.exp b/gdb/testsuite/gdb.base/tracefork-zombie.exp
index 03f790d4c5d..3e2e5517d46 100644
--- a/gdb/testsuite/gdb.base/tracefork-zombie.exp
+++ b/gdb/testsuite/gdb.base/tracefork-zombie.exp
@@ -58,8 +58,10 @@ foreach procpid [glob -directory /proc -type d {[0-9]*}] {
     if {[catch {open $procpid/status} statusfi]} {
 	continue
     }
-    set status [read $statusfi]
-    close $statusfi
+    if {[catch {read $statusfi} status]} {
+	continue
+    }
+    catch {close $statusfi}
     if {1
          && [regexp -line {^Name:\tgdb$} $status]
          && [regexp -line {^PPid:\t1$} $status]