gdbserver: check for nullptr condition in regcache::get_register_status

A regcache can be initialized with a register value buffer, in which
case, the register_status pointer is null.  This condition is checked
in set_register_status, but not in get_register_status.  Do this check
for consistence and safety.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
diff --git a/gdbserver/regcache.cc b/gdbserver/regcache.cc
index 583b562..4a064f6 100644
--- a/gdbserver/regcache.cc
+++ b/gdbserver/regcache.cc
@@ -526,7 +526,10 @@
 {
 #ifndef IN_PROCESS_AGENT
   gdb_assert (regnum >= 0 && regnum < tdesc->reg_defs.size ());
-  return (enum register_status) (register_status[regnum]);
+  if (register_status != nullptr)
+    return (enum register_status) (register_status[regnum]);
+  else
+    return REG_VALID;
 #else
   return REG_VALID;
 #endif