[PHP] Investigate and fix valgrind failures with PHP 8.1 prototype

[mfulb]

When running the valgrind tests on the PHP 8.1 prototype agent (built against PHP 8.1 RC2 and RC4) the tests test_curl and test_curl_md fail.

Example of typical failure:

Use of uninitialised value of size 8
==7404== at 0x8D8345: nr_hashmap_fetch (util_hashmap.c:293)
==7404== by 0x8D7EEF: nr_hashmap_get_into (util_hashmap.c:163)
==7404== by 0x8D7E3A: nr_hashmap_get (util_hashmap.c:143)
==7404== by 0x86C270: nr_hashmap_index_get (util_hashmap.h:201)
==7404== by 0x86C9D6: nr_php_curl_md_set_outbound_headers (php_curl_md.c:175)
==7404== by 0x88E46C: nr_php_curl_setopt_post (php_curl.c:473)
==7404== by 0x861C01: _nr_inner_wrapper_function_curl_setopt (php_internal_instrument.c:2167)
==7404== by 0x85D872: nr_php_instrument_delegate (php_internal_instrument.c:111)
==7404== by 0x8649B7: _nr_outer_wrapper_function_curl_setopt (php_internal_instrument.c:3040)
==7404== by 0x487CBE: zend_call_function (zend_execute_API.c:908)
==7404== by 0x4882D4: _call_user_function_impl (zend_execute_API.c:696)
==7404== by 0x88F4F5: nr_php_call_user_func (php_call.c:57)
==7404== by 0x88D9BE: nr_php_curl_set_default_request_headers (php_curl.c:164)
==7404== by 0x88DA2F: nr_php_curl_init (php_curl.c:177)
==7404== by 0x861D4B: _nr_inner_wrapper_function_curl_init (php_internal_instrument.c:2211)
==7404== by 0x85D872: nr_php_instrument_delegate (php_internal_instrument.c:111)
==7404== by 0x864A11: _nr_outer_wrapper_function_curl_init (php_internal_instrument.c:3042)
==7404== by 0x487CBE: zend_call_function (zend_execute_API.c:908)
==7404== by 0x4882D4: _call_user_function_impl (zend_execute_API.c:696)
==7404== by 0x88F4F5: nr_php_call_user_func (php_call.c:57)
==7404== by 0x467ED8: test_curl_metadata_get (test_curl_md.c:26)
==7404== by 0x4698F7: test_main (test_curl_md.c:466)
==7404== by 0x8584C0: test_main_parallel_driver_helper (tlib_main.c:189)
==7404== by 0x858657: test_main_parallel_driver (tlib_main.c:236)
==7404== by 0x858281: main (tlib_main.c:142)

Part of #258.

Acceptance Tasks:

• Investigate PHP 8.1 RC5 and record findings
• Investigate PHP 8.1 RC6 and record findings
• Investigate PHP 8.1 FINAL RELEASE and correct valgrind failures

[mfulb]

Rerunning the valgrind tests using PHP 8.1 RC5 with Ubuntu 16.04 and 20.04 hosts verified these errors still occur.

[mfulb]

Failures occurs with PHP 8.1 RC6 as well.