fix the scope for oauthbearer OIDC

[jliunyu]

There is a regression for oauthbearer oidc related to scope.

when malloc post_fields, the size is post_fields_size + 1, but when copy the string, the +1 missed. which is different than before. If the scope is “kirk-scope”, token provider will receive “kirk-scop”, the last character will be missed.

Local test with AP 3.1
ailing with expired JWT: PASS (10.09s) ]
[0126_oauthbearer_oidc       / 30.075s] 0126_oauthbearer_oidc: duration 30074.752ms
[0126_oauthbearer_oidc       / 30.075s] ================= Test 0126_oauthbearer_oidc PASSED =================
[<MAIN>                      / 30.099s] ALL-TESTS: duration 30098.795ms
TEST 20220714092549 (bare, scenario default) SUMMARY
#==================================================================#
| <MAIN>                                   |     PASSED |  30.099s |
| 0126_oauthbearer_oidc                    |     PASSED |  30.075s |
#==================================================================#

Unit test:

RDUT: INFO: rdkafka_sasl_oauthbearer_oidc.c:442: ut_sasl_oauthbearer_oidc_should_succeed: BEGIN: 
RDUT: PASS: rdkafka_sasl_oauthbearer_oidc.c:481: ut_sasl_oauthbearer_oidc_should_succeed
RDUT: INFO: rdkafka_sasl_oauthbearer_oidc.c:497: ut_sasl_oauthbearer_oidc_with_empty_key: BEGIN: 
RDUT: PASS: rdkafka_sasl_oauthbearer_oidc.c:527: ut_sasl_oauthbearer_oidc_with_empty_key
RDUT: INFO: rdkafka_sasl_oauthbearer_oidc.c:545: ut_sasl_oauthbearer_oidc_post_fields: BEGIN: 
RDUT: PASS: rdkafka_sasl_oauthbearer_oidc.c:560: ut_sasl_oauthbearer_oidc_post_fields
RDUT: INFO: rdkafka_sasl_oauthbearer_oidc.c:578: ut_sasl_oauthbearer_oidc_post_fields_with_empty_scope: BEGIN: 
RDUT: PASS: rdkafka_sasl_oauthbearer_oidc.c:593: ut_sasl_oauthbearer_oidc_post_fields_with_empty_scope
RDUT: INFO: rdunittest.c:502: rd_unittest: unittest: sasl_oauthbearer_oidc: PASS
[0000_unittests              /  3.443s] 0000_unittests: duration 3442.826ms
[0000_unittests              /  3.443s] ================= Test 0000_unittests PASSED =================
[<MAIN>                      /  4.016s] ALL-TESTS: duration 4015.695ms
TEST 20220714092931 (bare, scenario default) SUMMARY

[mhowlett]

maybe document these two as @param i think?

[mhowlett]

✅