runtime: check sched_getaffinity return value
runtime: check sched_getaffinity return value

Android on ChromeOS uses a restrictive seccomp filter that blocks
sched_getaffinity, leading this code to index a slice by -errno.

Change-Id: Iec09a4f79dfbc17884e24f39bcfdad305de75b37
Reviewed-on: https://go-review.googlesource.com/34794
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

josharian/go,ImJasonH/go,vsekhar/elastic-go,CAFxX/go,golang/go,ImJasonH/go,derekmarcotte/go,codestation/go,nawawi/go,ImJasonH/go,Cofyc/go,golang/go,golang/go,akutz/go,cloudflare/go,codestation/go,akutz/go,polyverse-security/go,ImJasonH/go,golang/go,golang/go,Cofyc/go,Cofyc/go,suncycheng/go,akutz/go,linux-on-ibm-z/go,linux-on-ibm-z/go,vsekhar/elastic-go,josharian/go,josharian/go,linux-on-ibm-z/go,nawawi/go,vsekhar/elastic-go,vsekhar/elastic-go,vsekhar/elastic-go,linux-on-ibm-z/go,CAFxX/go,christopher-henderson/Go,codestation/go,cloudflare/go,polyverse-security/go,akutz/go,linux-on-ibm-z/go,josharian/go,polyverse-security/go,akutz/go,josharian/go,golang/go,josharian/go,codestation/go,ImJasonH/go,nawawi/go,nawawi/go,derekmarcotte/go,suncycheng/go,derekmarcotte/go,akutz/go,cloudflare/go,polyverse-security/go,ImJasonH/go,CAFxX/go,Cofyc/go,josharian/go,nawawi/go,polyverse-security/go,suncycheng/go,golang/go,Cofyc/go,Cofyc/go,Cofyc/go,vsekhar/elastic-go,cloudflare/go,polyverse-security/go,codestation/go,ImJasonH/go,linux-on-ibm-z/go,linux-on-ibm-z/go,derekmarcotte/go,codestation/go,derekmarcotte/go,nawawi/go,CAFxX/go,suncycheng/go,CAFxX/go,CAFxX/go,cloudflare/go,suncycheng/go,CAFxX/go,suncycheng/go,suncycheng/go,vsekhar/elastic-go,Cofyc/go,codestation/go,nawawi/go,codestation/go,derekmarcotte/go,polyverse-security/go,derekmarcotte/go,cloudflare/go,derekmarcotte/go,linux-on-ibm-z/go,akutz/go,cloudflare/go,suncycheng/go,polyverse-security/go,cloudflare/go,nawawi/go,ImJasonH/go,golang/go,vsekhar/elastic-go,josharian/go,akutz/go,CAFxX/go