Merge bitcoin-core/secp256k1#969: ci: Fixes after Debian release

5d5c74a tests: Rewrite code to circument potential bug in clang (Tim Ruffing)
3d2f492 ci: Install libasan6 (instead of 5) after Debian upgrade (Tim Ruffing)

Pull request description:

ACKs for top commit:
  jonasnick:
    ACK 5d5c74a

Tree-SHA512: 540ede482214bf9feaa607de52a69f6d34169dd98fb14bd3d003f4c8f722c1eebed56eb9d933e742f36d8886c25bfa9fa0ebbed5b0c3b161f04dc26180f5d214

ci/linux-debian.Dockerfile

@@ -14,7 +14,7 @@ RUN apt-get install --no-install-recommends --no-upgrade -y \
         make automake libtool pkg-config dpkg-dev valgrind qemu-user \
         gcc clang llvm libc6-dbg \
         g++ \
-        gcc-i686-linux-gnu libc6-dev-i386-cross libc6-dbg:i386 libubsan1:i386 libasan5:i386 \
+        gcc-i686-linux-gnu libc6-dev-i386-cross libc6-dbg:i386 libubsan1:i386 libasan6:i386 \
         gcc-s390x-linux-gnu libc6-dev-s390x-cross libc6-dbg:s390x \
         gcc-arm-linux-gnueabihf libc6-dev-armhf-cross libc6-dbg:armhf \
         gcc-aarch64-linux-gnu libc6-dev-arm64-cross libc6-dbg:arm64 \

src/tests.c

@@ -5262,17 +5262,19 @@ void test_ecdsa_sign_verify(void) {
     secp256k1_scalar msg, key;
     secp256k1_scalar sigr, sigs;
     int getrec;
-    /* Initialize recid to suppress a false positive -Wconditional-uninitialized in clang.
-       VG_UNDEF ensures that valgrind will still treat the variable as uninitialized. */
-    int recid = -1; VG_UNDEF(&recid, sizeof(recid));
+    int recid;
     random_scalar_order_test(&msg);
     random_scalar_order_test(&key);
     secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &pubj, &key);
     secp256k1_ge_set_gej(&pub, &pubj);
     getrec = secp256k1_testrand_bits(1);
-    random_sign(&sigr, &sigs, &key, &msg, getrec?&recid:NULL);
+    /* The specific way in which this conditional is written sidesteps a potential bug in clang.
+       See the commit messages of the commit that introduced this comment for details. */
     if (getrec) {
+        random_sign(&sigr, &sigs, &key, &msg, &recid);
         CHECK(recid >= 0 && recid < 4);
+    } else {
+        random_sign(&sigr, &sigs, &key, &msg, NULL);
     }
     CHECK(secp256k1_ecdsa_sig_verify(&ctx->ecmult_ctx, &sigr, &sigs, &pub, &msg));
     secp256k1_scalar_set_int(&one, 1);