Merge bitcoin#889: fix uninitialized read in tests

99a1cfe print warnings for conditional-uninitialized (PiRK)
3d2cf6c initialize variable in tests (PiRK)

Pull request description:

ACKs for top commit:
  real-or-random:
    ACK 99a1cfe code inspection
  jonasnick:
    ACK 99a1cfe

Tree-SHA512: 72f92f51c44210ab54f166920f540525db0e3d1f19a2fa56e4a6d157a38a582f9dc649d919cf3278482c9fd723021b07759284a8fccbc574b62a22aac0facf51

configure.ac

@@ -82,6 +82,15 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char foo;]])],
       CFLAGS="$saved_CFLAGS"
     ])
 
+saved_CFLAGS="$CFLAGS"
+CFLAGS="-Wconditional-uninitialized $CFLAGS"
+AC_MSG_CHECKING([if ${CC} supports -Wconditional-uninitialized])
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char foo;]])],
+    [ AC_MSG_RESULT([yes]) ],
+    [ AC_MSG_RESULT([no])
+      CFLAGS="$saved_CFLAGS"
+    ])
+
 saved_CFLAGS="$CFLAGS"
 CFLAGS="-fvisibility=hidden $CFLAGS"
 AC_MSG_CHECKING([if ${CC} supports -fvisibility=hidden])

src/tests.c

@@ -4526,8 +4526,10 @@ void test_ecdsa_sign_verify(void) {
     secp256k1_scalar one;
     secp256k1_scalar msg, key;
     secp256k1_scalar sigr, sigs;
-    int recid;
     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));
     random_scalar_order_test(&msg);
     random_scalar_order_test(&key);
     secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &pubj, &key);