cmd/pprof: don't use offset if we don't have a start address
The test is in the runtime package because there are other tests of
pprof there. At some point we should probably move them all into a pprof
testsuite.
Fixes #16128.
Change-Id: Ieefa40c61cf3edde11fe0cf04da1debfd8b3d7c0
Reviewed-on: https://go-review.googlesource.com/24274
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Andrew Gerrand <adg@golang.org>