missing/setproctitle.c: remove nonsense NULL check

If fmt is NULL, ptitle is uninitialized and used.
SETPROCTITLE(3bsd) says "If fmt is NULL, the process title is restored",
but looks like the feature is not implemented in missing/setproctitle.c.
At least the source code of ruby does not pass NULL to the function.
So I assume this function requires non-NULL fmt.

This issue was found by Coverity Scan.

missing/setproctitle.c

@@ -152,10 +152,9 @@ setproctitle(const char *fmt, ...)
 		return;
 #endif
 
+	/* fmt must be non-NULL */
 	va_start(ap, fmt);
-	if (fmt != NULL) {
-		vsnprintf(ptitle, sizeof(ptitle) , fmt, ap);
-	}
+	vsnprintf(ptitle, sizeof(ptitle), fmt, ap);
 	va_end(ap);
 
 #if SPT_TYPE == SPT_PSTAT