Fix nil bug in generic/custom and change opt order

I think PAYLOADSTR should take precedence over PAYLOADFILE. Usually,
you'll use PAYLOADFILE but might want to override with PAYLOADSTR. I
doubt this change will hurt anyone, since few people set both at once.
The payload description even says "either," so there's that.

Author: wvu

modules/payloads/singles/generic/custom.rb

@@ -42,10 +42,12 @@ def generate
       self.arch = actual_arch
     end
 
-    if datastore['PAYLOADFILE']
-      IO.read(datastore['PAYLOADFILE'])
-    elsif datastore['PAYLOADSTR']
+    if datastore['PAYLOADSTR']
       datastore['PAYLOADSTR']
+    elsif datastore['PAYLOADFILE']
+      IO.read(datastore['PAYLOADFILE'])
+    else
+      ''
     end
   end