Fix GDB crash when registers cannot be modified.

This crash was detected when using GDB with the valgrind gdbserver.
To reproduce:

valgrind sleep 10000

In another window:
gdb
target remote | vgdb
p printf("make sleep print something\n")
=>
terminate called after throwing an instance of 'gdb_exception_error'
Aborted

The problem is that the valgrind gdbserver does not allow to change
registers when the inferior is blocked in a system call.
GDB then raises an exception.  The exception causes the destructor
of
 typedef std::unique_ptr<infcall_suspend_state, infcall_suspend_state_deleter>
    infcall_suspend_state_up;
to be called.  This destructor itself tries to restore the value of
the registers, and fails similarly.  We must catch the exception in
the destructor to avoid crashing GDB.
If the destructor encounters a problem, no warning is produced if
there is an uncaught exception, as in this case, the user will already
be informed of a problem via this exception.

With this change, no crash anymore, and all the valgrind 3.15 tests
pass succesfully.

gdb/ChangeLog
2019-04-19  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* inferior.h (struct infcall_suspend_state_deleter):
	Catch exception in destructor to avoid crash.

gdb/inferior.h

@@ -68,7 +68,18 @@ struct infcall_suspend_state_deleter
 {
   void operator() (struct infcall_suspend_state *state) const
   {
-    restore_infcall_suspend_state (state);
+    try
+      {
+	restore_infcall_suspend_state (state);
+      }
+    catch (const gdb_exception_error &e)
+      {
+	/* If we are restoring the inferior state due to an exception,
+	   some error message will be printed.  So, only warn the user
+	   when we cannot restore during normal execution.  */
+	if (!std::uncaught_exception ())
+	  warning (_("Failed to restore inferior state: %s"), e.what ());
+      }
   }
 };