runtime/metrics: fix panic in Read with empty slice

Calling Read with a nil or empty slice previously caused a panic with
"index out of range" because the function unconditionally accessed the
first element of the slice (via &m[0]) to pass the pointer to the
runtime.

This change adds a check for len(m) == 0 to return early, preventing
the panic when no samples are provided.

Fixes #77231

Change-Id: I442635f5c61de432883c8d0efae9cc6aa1363cc9
GitHub-Last-Rev: 6f24f67b18c77a0b36b92017a3f4ef8aa3aa5229
GitHub-Pull-Request: golang/go#77233
Reviewed-on: https://go-review.googlesource.com/c/go/+/737380
Reviewed-by: Amol Yadav <amolyadav6125@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Commit-Queue: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

0 files changed, 0 insertions, 0 deletions