Fix crash when -R option is not provided

Author: Martin Vejnár

pe_tools/peresed.py

@@ -61,7 +61,7 @@ def main():
     gp.add_argument('--add-dependency', '-M', action='append', metavar="DEP", default=[], help="add dependency. DEP should be a space separated list of key=value pairs, e.g. " +
         "\"type=win32 name=Microsoft.Windows.Common-Controls version=6.0.0.0 processorArchitecture=* publicKeyToken=6595b64144ccf1df language=*\"")
     gp.add_argument('--set-version', '-V', action='append', metavar="STR", help="updates the specified version-info field, e.g. FileVersion=\"1, 2, 3, 4\"")
-    gp.add_argument('--set-resource', '-R', metavar=('TYPE', 'NAME', 'LANG', 'FILE'), nargs=4, action='append', help='set a resource entry to the contents of a file, e.g. "-R RT_RCDATA prog.exe 0 prog.exe"')
+    gp.add_argument('--set-resource', '-R', metavar=('TYPE', 'NAME', 'LANG', 'FILE'), nargs=4, action='append', default=[], help='set a resource entry to the contents of a file, e.g. "-R RT_RCDATA prog.exe 0 prog.exe"')
 
     ap.add_argument('--output', '-o', help="write the edited contents to OUTPUT instead of editing the input file in-place")
     ap.add_argument('file', help="the PE file to parse and edit")