tryexec() in ash relies on all callers of shellexec() having an additional unused element before the start of its argv array. In busybox-w32 this was not the case when shellexec() was called from forkshell_shellexec(), as only the actual contents of the argv array were copied into the forkshell data block. In practice argv[-1] is only currently used when the 'which' applet is about to be run, so whatever got overwritten (probably cmdtable[30]) was unlikely to matter. Still, let's be correct and allocate the additional element. Adds 16 bytes.