Don't overwrite STR (aka bbs.command_str) in EXEC_BIN unless an actual argument

was supplied. This solves the problem of "exec_bin str_cmds" not working with
str_cmds.js, since bbs.command_str was being erased if the %s argument wasn't
included in the command-line.

src/sbbs3/exec.cpp

@@ -671,7 +671,8 @@ long sbbs_t::exec_bin(const char *cmdline, csi_t *csi)
 		p++;				/* skip space */
 		SKIP_CHAR(p,' ');	/* skip more spaces */
 	}
-	strcpy(main_csi.str, p);
+	if(*p)
+		strcpy(main_csi.str, p);
 
 #ifdef JAVASCRIPT
 	if((p=getfext(mod))!=NULL && stricmp(p,".js")==0)