cmd/internal/obj: don't crash on nil in Prog.String
cmd/internal/obj: don't crash on nil in Prog.String

I can't remember just how this happened to me, but I got an unfortunate
crash with some set of cmd/compile debug options and source code.

Change-Id: Ibef6129c50b68dad0594ac439466bfbc4b32a095
Reviewed-on: https://go-review.googlesource.com/19920
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

Cofyc/go,momchil-velikov/go,mwhudson/go,goodlang/good,linux-on-ibm-z/go,suncycheng/go,binarycrusader/go,codestation/go,linux-on-ibm-z/go,vanloswang/go,codestation/go,ganboing/go-esx,akutz/go,golang/go,vsekhar/elastic-go,christopher-henderson/Go,binarycrusader/go,derekmarcotte/go,theif519/go,linux-on-ibm-z/go,akutz/go,ganboing/go-esx,josharian/go,momchil-velikov/go,codestation/go,derekmarcotte/go,codestation/go,josharian/go,Samurais/go,derekmarcotte/go,cloudflare/go,goodlang/good,codestation/go,nawawi/go,josharian/go,goodlang/good,miolini/go,goodlang/good,golang/go,Cofyc/go,derekmarcotte/go,binarycrusader/go,miolini/go,ganboing/go-esx,mwhudson/go,theif519/go,4ad/go,suncycheng/go,vsekhar/elastic-go,4ad/go,CAFxX/go,miolini/go,CAFxX/go,suncycheng/go,golang/go,Cofyc/go,ganboing/go-esx,momchil-velikov/go,golang/go,momchil-velikov/go,Samurais/go,Cofyc/go,Samurais/go,4ad/go,nawawi/go,Cofyc/go,ganboing/go-esx,mwhudson/go,Samurais/go,4ad/go,ImJasonH/go,cloudflare/go,mwhudson/go,momchil-velikov/go,ImJasonH/go,4ad/go,binarycrusader/go,polyverse-security/go,ImJasonH/go,vsekhar/elastic-go,mwhudson/go,josharian/go,theif519/go,suncycheng/go,akutz/go,nawawi/go,akutz/go,vanloswang/go,binarycrusader/go,vsekhar/elastic-go,theif519/go,codestation/go,nawawi/go,mwhudson/go,cloudflare/go,ganboing/go-esx,codestation/go,4ad/go,ImJasonH/go,Cofyc/go,ganboing/go-esx,ImJasonH/go,momchil-velikov/go,CAFxX/go,akutz/go,suncycheng/go,polyverse-security/go,suncycheng/go,theif519/go,vanloswang/go,miolini/go,CAFxX/go,ImJasonH/go,nawawi/go,ganboing/go-esx,golang/go,goodlang/good,polyverse-security/go,derekmarcotte/go,vsekhar/elastic-go,theif519/go,josharian/go,vanloswang/go,mwhudson/go,polyverse-security/go,Cofyc/go,miolini/go,akutz/go,vsekhar/elastic-go,theif519/go,cloudflare/go,goodlang/good,Samurais/go,josharian/go,theif519/go,miolini/go,derekmarcotte/go,CAFxX/go,ImJasonH/go,linux-on-ibm-z/go,cloudflare/go,CAFxX/go,golang/go,goodlang/good,golang/go,polyverse-security/go,CAFxX/go,nawawi/go,nawawi/go,codestation/go,binarycrusader/go,Samurais/go,momchil-velikov/go,vsekhar/elastic-go,miolini/go,cloudflare/go,polyverse-security/go,goodlang/good,vanloswang/go,vanloswang/go,linux-on-ibm-z/go,suncycheng/go,linux-on-ibm-z/go,akutz/go,Samurais/go,binarycrusader/go,akutz/go,CAFxX/go,josharian/go,suncycheng/go,4ad/go,josharian/go,cloudflare/go,linux-on-ibm-z/go,Samurais/go,linux-on-ibm-z/go,Cofyc/go,vanloswang/go,4ad/go,derekmarcotte/go,derekmarcotte/go,polyverse-security/go,nawawi/go,binarycrusader/go,golang/go,mwhudson/go,momchil-velikov/go,vsekhar/elastic-go,polyverse-security/go,cloudflare/go,ImJasonH/go,miolini/go,vanloswang/go