Fixed a bug in sstrerror()

I introduced a bug into sstrerror() when I added error checking code
around the call to Cglobals_get().  The hash of the offending commit is:

ff8021a2

Before my so say improvements, the return value of Cglobals_get() was
not checked.  I added a check that assumed an error had occured if the
return value was non-zero.  Unfortuntaley for me, the return values 0
and 1 both mean success.  A return value of -1 signifies a failure.

mediachanger/castorrmc/common/serror.c

@@ -626,7 +626,7 @@ const char *sstrerror(const int n) {
   void *buf = NULL;
   int buflen = 80;
 
-  if(Cglobals_get(&sstrerror_key, &buf, buflen)) {
+  if(0 > Cglobals_get(&sstrerror_key, &buf, buflen)) {
     return "Unknown error"
       ": No thread specific memory for error string"
       ": Cglobals_get() failed";