As reported by Leo and then analyzed by Mark, grafting crashes when the
replacement string lies near the end of buffer:

  https://lists.gnu.org/archive/html/guix-devel/2017-07/msg00445.html

With the attached patch on top of commit
7d14082d56462f7bef4254d65a21fd265fbce471, I get this backtrace:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix build go@1.9

[...]

@ build-started /gnu/store/n420a0qs7vwnbaj4j3knhfy9j7zqwvs4-go-1.9rc1.drv - x86_64-linux /var/log/guix/drvs/n4//20a0qs7vwnbaj4j3knhfy9j7zqwvs4-go-1.9rc1.drv.bz2
grafting '/gnu/store/8zbi5rr5q2pxksfgpxq37m60nsa21d3v-go-1.9rc1-doc' -> '/gnu/store/djdi6b1bvbrp5fgkkrvk80jndg6gifrh-go-1.9rc1-doc'...
grafting '/gnu/store/73fvsx0qizcrbajgjwn8mkzkpxa6abx4-go-1.9rc1' -> '/gnu/store/8bjpc7wxdxsxbf45qvwsd6jbn7zgi871-go-1.9rc1'...
Backtrace:
In ice-9/boot-9.scm:
 160: 17 [catch #t #<catch-closure 8c5dc0> ...]
In unknown file:
   ?: 16 [apply-smob/1 #<catch-closure 8c5dc0>]
In ice-9/boot-9.scm:
  66: 15 [call-with-prompt prompt0 ...]
In ice-9/eval.scm:
 432: 14 [eval # #]
In ice-9/boot-9.scm:
2412: 13 [save-module-excursion #<procedure 8e6840 at ice-9/boot-9.scm:4084:3 ()>]
4089: 12 [#<procedure 8e6840 at ice-9/boot-9.scm:4084:3 ()>]
1734: 11 [%start-stack load-stack #<procedure 8f6e20 at ice-9/boot-9.scm:4080:10 ()>]
1739: 10 [#<procedure 8f8960 ()>]
In unknown file:
   ?: 9 [primitive-load "/gnu/store/zslsp0k0m06ab7a2vzf2wp3cv6s5zwxn-go-1.9rc1-guile-builder"]
In ice-9/eval.scm:
 387: 8 [eval # ()]
In ice-9/boot-9.scm:
 806: 7 [for-each #<procedure c44b10 at ice-9/eval.scm:416:20 (a b)> # #]
In /gnu/store/hnwfmnrb1r7dzajn744dafx19ps290d8-module-import/guix/build/graft.scm:
 313: 6 [rewrite-directory "/gnu/store/73fvsx0qizcrbajgjwn8mkzkpxa6abx4-go-1.9rc1" ...]
In srfi/srfi-1.scm:
 616: 5 [for-each #<procedure rewrite-leaf (file)> #]
In ice-9/boot-9.scm:
 877: 4 [call-with-input-file "/gnu/store/73fvsx0qizcrbajgjwn8mkzkpxa6abx4-go-1.9rc1/pkg/tool/linux_amd64/link" ...]
 895: 3 [call-with-output-file "/gnu/store/8bjpc7wxdxsxbf45qvwsd6jbn7zgi871-go-1.9rc1/pkg/tool/linux_amd64/link" ...]
In /gnu/store/hnwfmnrb1r7dzajn744dafx19ps290d8-module-import/guix/build/graft.scm:
 302: 2 [#<procedure 13d45c0 at /gnu/store/hnwfmnrb1r7dzajn744dafx19ps290d8-module-import/guix/build/graft.scm:301:15 (output)> #<output: /gnu/store/8bjpc7wxdxsxbf45qvwsd6jbn7zgi871-go-1.9rc1/pkg/tool/linux_amd64/link 7>]
 173: 1 [replace-store-references #<input: /gnu/store/73fvsx0qizcrbajgjwn8mkzkpxa6abx4-go-1.9rc1/pkg/tool/linux_amd64/link 6> ...]
In unknown file:
   ?: 0 [put-bytevector #<output: /gnu/store/8bjpc7wxdxsxbf45qvwsd6jbn7zgi871-go-1.9rc1/pkg/tool/linux_amd64/link 7> ...]

ERROR: In procedure put-bytevector:
ERROR: In procedure put-bytevector: Value out of range: 0
builder for `/gnu/store/n420a0qs7vwnbaj4j3knhfy9j7zqwvs4-go-1.9rc1.drv' failed with exit code 1
@ build-failed /gnu/store/n420a0qs7vwnbaj4j3knhfy9j7zqwvs4-go-1.9rc1.drv - 1 builder for `/gnu/store/n420a0qs7vwnbaj4j3knhfy9j7zqwvs4-go-1.9rc1.drv' failed with exit code 1
--8<---------------cut here---------------end--------------->8---

Ludo’.