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/ChangeLog

@@ -77,6 +77,11 @@
 	* source.c (print_source_lines_base): Advance "iter" when a
 	control character is seen.
 
+2019-04-19  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
+
+	* inferior.h (struct infcall_suspend_state_deleter):
+	Catch exception in destructor to avoid crash.
+
 2019-04-19  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
 
 	* cli/cli-cmds.c (_initialize_cli_cmds): Move "shell" "!" alias