ash: exec: Do not allocate stack string in padvance

ash: exec: Do not allocate stack string in padvance

Upstream commit:

    Date: Sat, 19 May 2018 02:39:48 +0800
    exec: Do not allocate stack string in padvance

    Many callers of padvance immediately free the allocated string
    so this patch moves the stalloc call to the caller.  Instead of
    returning the allocated string, padvance now returns the length
    to allocate (this may be longer than the actual string length,
    even including the NUL).  For the case where we would previously
    return NULL, we now return -1.

    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>