Check for argc==0 when calling wtokensplit()

The WINGs-function wtokensplit does not set argv to NULL if no string has been
split - instead argc is set to 0.

You can only observe this issue if you compile Window Maker without any
optimization:

compile Window Maker with CFLAGS=""
run Window Maker and save a session
change session-file and replace a 'command xyz' line with 'command " "'
restart Window Maker
watch "Fatal error"-message

Retrieved-from: http://paldium.homeunix.org/tobias/wmaker/
Submitted-by: Gilbert Ashley <amigo@ibiblio.org>

src/dock.c

@@ -2977,7 +2977,7 @@ execCommand(WAppIcon *btn, char *command, WSavedState *state)
 
     wtokensplit(cmdline, &argv, &argc);
 
-    if (argv==NULL) {
+    if (!argc) {
         if (cmdline)
             wfree(cmdline);
         if (state)

src/session.c

@@ -381,7 +381,7 @@ execCommand(WScreen *scr, char *command, char *host)
 
     wtokensplit(command, &argv, &argc);
 
-    if (argv==NULL) {
+    if (!argc) {
         return 0;
     }