Fix memory corruption issue when executing context queries in RAM/SAVE memory mode

[stelfrag]

Fixes #10929

Summary

Then executing queries in RAM / SAVE mode, the metric_uuid was never properly allocated and
would result in memory corruption.

Component Name

database
web

Test Plan

• Start agent with memory mode not being dbengine e.g RAM / SAVE
• Execute a query http://127.0.0.1:19999/api/v1/data?context=net.net
  • This will attempt to free a copy of a rd->state->metric_uuid that belongs to the original rrddim and result to a crash
• Apply the PR
• Agent should not crash

Additional Information

Note: This is a temporary fix. A new PR in the next version avoids the memory allocation of the metric_uuid

[thiagoftsm]

I tested locally and on cloud and netdata did not crash when I ran netdata using ram.