[libmicrohttpd] PATCH: Avoid unitialized variable compiler warning in digestauth.c

[Thorsten Brehm]

Hi,

gcc10 complains with two warnings when compiling libmicrohttpd using

    #define NDEBUG 1
    #define DAUTH_SUPPORT 1

so, "release build" with enabled "digest authentication":

../digestauth.c: In function 'MHD_digest_auth_check_digest2':
../digestauth.c:1287:9: warning: 'da.digest_size' may be used uninitialized in 
this function [-Wmaybe-uninitialized]
 1287 |   if (da.digest_size != digest_size)
      |       ~~^~~~~~~~~~~~
../digestauth.c: In function 'MHD_queue_auth_fail_response2':
../digestauth.c:1361:55: warning: 'da.digest_size' may be used uninitialized in 
this function [-Wmaybe-uninitialized]
 1361 |     char nonce[NONCE_STD_LEN (VLA_ARRAY_LEN_DIGEST (da.digest_size)) + 
1];
      | 
      
This is a minor issue, without any practical effect, unless when calling the 
MHD API with an invalid value for the MHD_DigestAuthAlgorithm enum.
However, gcc is still right that there is a potential code path with undefined 
behaviour: the default-case in the switch statement in SETUP_DA does not set 
"da.digest_size".
Two functions later still always read this value. And the "mhd_assert" has no 
effect, since it's disabled when NDEBUG is set.

Trivial patch attached to silence the compiler warnings by also initializing 
"da.digest_size" in the default case of the switch statement:

diff --git a/src/microhttpd/digestauth.c b/src/microhttpd/digestauth.c
index 04c9f22a..aaed170e 100644
--- a/src/microhttpd/digestauth.c
+++ b/src/microhttpd/digestauth.c
@@ -1215,6 +1215,7 @@ MHD_digest_auth_check (struct MHD_Connection *connection,
       da.digest = &MHD_SHA256_finish;                         \
       break;                                              \
     default:                                              \
+      da.digest_size = 0;                                 \
       mhd_assert (false);                                 \
       break;                                              \
     }                                                     \

Best Regards,
Thorsten