Pattern match the map key for more efficiency

[NelsonVides]

Doing maps:find creates the ok-tuple, which means garbage, and also then
needs to match on it. It it more efficient to pattern-match on the map key.

[codecov]

Codecov Report

Merging #3635 (9527a06) into master (6d1c24b) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3635      +/-   ##
==========================================
- Coverage   81.03%   81.03%   -0.01%     
==========================================
  Files         427      427              
  Lines       31962    31962              
==========================================
- Hits        25901    25899       -2     
- Misses       6061     6063       +2     

Impacted Files	Coverage Δ
src/mongoose_acc.erl	92.64% <100.00%> (ø)
...system_metrics/service_mongoose_system_metrics.erl	78.43% <0.00%> (-7.85%)	⬇️
src/elasticsearch/mongoose_elasticsearch.erl	76.92% <0.00%> (-7.70%)	⬇️
src/mongoose_tcp_listener.erl	78.04% <0.00%> (-4.88%)	⬇️
src/mam/mod_mam_elasticsearch_arch.erl	85.08% <0.00%> (-1.76%)	⬇️
src/pubsub/mod_pubsub_db_mnesia.erl	92.43% <0.00%> (-0.43%)	⬇️
src/mod_muc_log.erl	63.21% <0.00%> (ø)
src/pubsub/mod_pubsub.erl	72.94% <0.00%> (+0.06%)	⬆️
src/mod_muc_room.erl	76.70% <0.00%> (+0.11%)	⬆️
src/inbox/mod_inbox_rdbms.erl	94.25% <0.00%> (+0.57%)	⬆️
... and 2 more

---

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[mongoose-im]

small_tests_24 / small_tests / 9527a06
Reports root / small

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small_tests_23 / small_tests / 9527a06
Reports root / small

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dynamic_domains_pgsql_mnesia_23 / pgsql_mnesia / 9527a06
Reports root/ big
OK: 2860 / Failed: 0 / User-skipped: 133 / Auto-skipped: 0

---

dynamic_domains_pgsql_mnesia_24 / pgsql_mnesia / 9527a06
Reports root/ big
OK: 2860 / Failed: 0 / User-skipped: 133 / Auto-skipped: 0

---

dynamic_domains_mysql_redis_24 / mysql_redis / 9527a06
Reports root/ big
OK: 2843 / Failed: 0 / User-skipped: 150 / Auto-skipped: 0

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dynamic_domains_mssql_mnesia_24 / odbc_mssql_mnesia / 9527a06
Reports root/ big
OK: 2860 / Failed: 0 / User-skipped: 133 / Auto-skipped: 0

---

ldap_mnesia_23 / ldap_mnesia / 9527a06
Reports root/ big
OK: 1506 / Failed: 0 / User-skipped: 401 / Auto-skipped: 0

---

ldap_mnesia_24 / ldap_mnesia / 9527a06
Reports root/ big
OK: 1506 / Failed: 0 / User-skipped: 401 / Auto-skipped: 0

---

internal_mnesia_24 / internal_mnesia / 9527a06
Reports root/ big
OK: 1547 / Failed: 0 / User-skipped: 360 / Auto-skipped: 0

---

pgsql_mnesia_23 / pgsql_mnesia / 9527a06
Reports root/ big
OK: 3234 / Failed: 0 / User-skipped: 142 / Auto-skipped: 0

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pgsql_mnesia_24 / pgsql_mnesia / 9527a06
Reports root/ big
OK: 3234 / Failed: 0 / User-skipped: 142 / Auto-skipped: 0

---

elasticsearch_and_cassandra_24 / elasticsearch_and_cassandra_mnesia / 9527a06
Reports root/ big
OK: 1854 / Failed: 0 / User-skipped: 368 / Auto-skipped: 0

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mysql_redis_24 / mysql_redis / 9527a06
Reports root/ big
OK: 3229 / Failed: 0 / User-skipped: 147 / Auto-skipped: 0

---

mssql_mnesia_24 / odbc_mssql_mnesia / 9527a06
Reports root/ big
OK: 3234 / Failed: 0 / User-skipped: 142 / Auto-skipped: 0

---

riak_mnesia_24 / riak_mnesia / 9527a06
Reports root/ big
OK: 1697 / Failed: 0 / User-skipped: 367 / Auto-skipped: 0

[chrzaszcz]

Looks good 👍