nfc_init() return rather than exit on malloc error, examples fixed accordingly

examples/pn53x-diagnose.c

@@ -69,6 +69,10 @@ main(int argc, const char *argv[])
 
   nfc_context *context;
   nfc_init(&context);
+  if (context == NULL) {
+    ERR("Unable to init libnfc (malloc)");
+    exit(EXIT_FAILURE);
+  }
 
   // Display libnfc version
   acLibnfcVersion = nfc_version();