saft-*: Handle empty device list for `-f`flag

[wirew0rm]

If the saftd service does not have any devices attached, Saftd_Proxy::create()->getDevices() will return an empty map. This change checks for this edge case before trying to create a TimingReceiverProxy which will fail with std::bad_alloc otherwise. With this change an error message is printed and an error code of -1 is returned, which matches the behaviour of calling saft-* <nonexistent device> ....

fixes #108

[nix-shell:~/timing]$ saft-ctl bla -fjks
devices attached on this host   : 1
  device: /de/gsi/saftlib/tr0, name: tr0, path: dev/wbm0, gatewareVersion : 6.1.2
  --gateware version info:
  ---- Mon Aug 09 08:48:31 CEST 2021
  ---- fallout-v6.1.2
  ---- Arria V (5agxma3d4f27i3)
  ---- CentOS Linux release 7.9.2009 (Core), kernel 3.10.0-1160.36.2.el7.x86_64
  ---- pexaria5 +db[12] +wrex1
  ---- Timing Group Jenkins <csco-tg@gsi.de>
  ---- tsl021.acc.gsi.de
  ---- pci_control
  ---- Version 18.1.0 Build 625 09/12/2018 SJ Standard Edition
  ---- fallout-3847

6.1.2
no WR lock!!!
receiver free conditions: 256, max (capacity of HW): 0(256), early threshold: 4294967296 ns, latency: 4096 ns
sinks instantiated on this host: 1
  /de/gsi/saftlib/tr0/software/_3 (minOffset: -1000000000 ns, maxOffset: 1000000000 ns)
  -- actions: 0, delayed: 0, conflict: 0, late: 0, early: 0, overflow: 0 (max signalRate: 10Hz)
  -- conditions: 0

[nix-shell:~/timing]$ saft-ctl tr0 remove
Device 'tr0' was removed

[nix-shell:~/timing]$ saft-ctl bla -fs  # this command would crash with std::bad_alloc without this change
No devices attached to saftd