Land rapid7#4143, @kernelsmith's get_custom_exe fix

* Initializes the exe variable
* Fixes rapid7#4131

Author: jvazquez-r7

lib/msf/core/exploit/exe.rb

@@ -38,10 +38,11 @@ def get_eicar_exe
     obfus_eicar.join("-").upcase
   end
 
-  def get_custom_exe(path=nil)
+  def get_custom_exe(path = nil)
     path ||= datastore['EXE::Custom']
     print_status("Using custom payload #{path}, RHOST and RPORT settings will be ignored!")
     datastore['DisablePayloadHandler'] = true
+    exe = nil
     ::File.open(path,'rb') {|f| exe = f.read(f.stat.size)}
     exe
   end
@@ -160,7 +161,7 @@ def exe_init_options(opts)
   end
 
   def exe_post_generation(opts)
-    if (opts[:fellback])
+    if opts[:fellback]
       print_status("Warning: Falling back to default template: #{opts[:fellback]}")
     end
   end