A previous commit tried to eliminate a panic when reading an empty
profile by stopping support for profiles with no samples. That is
unnecessary and it can obstruct some testing.

The panic was caused by the legacy parser returning a nil profile and
no error when processing an empty profile. Detect that situation and
generate a proper profile instead.

Tested by running 'pprof /dev/null'
Add testcase for profile parsing errors

Preserving the old ones while needed

Should have no functional changes

- Remove dead code, unnecessary conversions
- Consistently use a single space after period

Legacy profiles that are not based on interrupt-based sampling
only include addresses that are call stack return addresses. For
them, all callstack addresses should be adjusted by -1 to land
on top of the call instruction and permit accurate symbolization.

This fixes issue #9

The legacy profilez parser handles duplicate leaf samples that are a
common artifact of satck unwinding. Apply the same technique to threadz
profiles where duplicate samples also occur.

A Go redefinition error causes pprof to skip parsing profile mappings.

Also update the build instructions to use go get