[mod_amqp] prevent segfault on double connection close

[greenbea]

When a command response publish failed, mod_amqp_command_response closed the connection and cleared conn_active. The command thread teardown then called mod_amqp_connection_close(NULL), causing a segfault.

Here's the test I've used to reproduce this.
test.zip

Also fix amqp_error_string2() calls to pass status codes without erroneous negation.

Description

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Code cleanup / refactor

Related Issues

Testing

• Added/updated unit tests
• Tested manually
• Tested with live SignalWire credentials (if applicable)

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the project's style guidelines
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)
• All existing tests pass

Additional Notes

[greenbea]

segfault

Core was generated by `/usr/bin/freeswitch -u www-data -g www-data -ncwait -nonat'.
Program terminated with signal SIGSEGV, Segmentation fault.
--Type <RET> for more, q to quit, c to continue without paging--
#0  0x00007f16721ab085 in mod_amqp_connection_close (connection=0x0) at mod_amqp_connection.c:43.
43              amqp_connection_state_t old_state;
[Current thread is 1 (Thread 0x7f16721456c0 (LWP 1804290))]
(gdb) bt
#0  0x00007f16721ab085 in mod_amqp_connection_close (connection=0x0) at mod_amqp_connection.c:43
#1  0x00007f16721ade10 in mod_amqp_command_thread (thread=0x5619eb370498, data=<optimized out>) at mod_amqp_command.c:501
#2  0x00007f167a0a71f5 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
#3  0x00007f167a1278dc in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:62
#4  0x0000000000000000 in ?? ()