Fix inconsistency found by static analyzers

[Anchels]

Hello! I was analyzing Nginx modules with the Svace static analyzer. It has found an inconsistency code at the following sections of the code:

https://github.com/samizdatco/nginx-http-auth-digest/blob/5a2cae4812d8a1ba5f83dfbcb8d043d05c8e6f97/ngx_http_auth_digest_module.c#L1227-L1247

and

https://github.com/samizdatco/nginx-http-auth-digest/blob/5a2cae4812d8a1ba5f83dfbcb8d043d05c8e6f97/ngx_http_auth_digest_module.c#L1286-L1308

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In both methods the result value dropnode of method invocation ngx_array_push is dereferenced without checking for NULL:

https://github.com/samizdatco/nginx-http-auth-digest/blob/5a2cae4812d8a1ba5f83dfbcb8d043d05c8e6f97/ngx_http_auth_digest_module.c#L1245

and

https://github.com/samizdatco/nginx-http-auth-digest/blob/5a2cae4812d8a1ba5f83dfbcb8d043d05c8e6f97/ngx_http_auth_digest_module.c#L1306

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Here's the source code for function ngx_array_push:

https://github.com/nginx/nginx/blob/ecb809305e54ed15be9f620d56b19ff4e4be7db5/src/core/ngx_array.c#L47-L91

Note that this function can return NULL. Therefore, when using it, it is important to check the result for NULL in order to avoid possible errors. And as a rule, such check is performed in other modules that use this function.

What do you think about adding the NULL checks?

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Found by Linux Verification Center (linuxtesting.org) with SVACE.

[Anchels]

Similar issues, but with ngx_palloc function:

shm_name is dereferenced without checking for null: https://github.com/samizdatco/nginx-http-auth-digest/blob/5a2cae4812d8a1ba5f83dfbcb8d043d05c8e6f97/ngx_http_auth_digest_module.c#L75-L76

same for *d:

https://github.com/samizdatco/nginx-http-auth-digest/blob/5a2cae4812d8a1ba5f83dfbcb8d043d05c8e6f97/ngx_http_auth_digest_module.c#L499-L517

[LM4O322]

Looks like it might be worth adding a Null check herу too: HA2.data is dereferenced without checking for NULL: https://github.com/samizdatco/nginx-http-auth-digest/blob/5a2cae4812d8a1ba5f83dfbcb8d043d05c8e6f97/ngx_http_auth_digest_module.c#L716

HA1.data here: https://github.com/samizdatco/nginx-http-auth-digest/blob/5a2cae4812d8a1ba5f83dfbcb8d043d05c8e6f97/ngx_http_auth_digest_module.c#L675

and

auth_fields here: https://github.com/samizdatco/nginx-http-auth-digest/blob/5a2cae4812d8a1ba5f83dfbcb8d043d05c8e6f97/ngx_http_auth_digest_module.c#L286