Merge #1364: Avoid -Wmaybe-uninitialized when compiling with gcc -O1

5b9f37f ci: Add `CFLAGS: -O1` to task matrix (Hennadii Stepanov)
a6ca76c Avoid `-Wmaybe-uninitialized` when compiling with `gcc -O1` (Hennadii Stepanov)

Pull request description:

  Fixes #1361.

  CI tasks have been adjusted to catch similar issues in the future.

ACKs for top commit:
  real-or-random:
    utACK 5b9f37f
  jonasnick:
    tACK 5b9f37f

Tree-SHA512: 8aa5ec22ed88579ecd37681df68d64f8bab93cd14bdbf432a3af41cadc7ab3eba86c33c179db15bf3a3c798c33064bd845ebdedb02ee617ef634e98c596838c2

Author: real-or-random

.cirrus.yml

@@ -80,6 +80,7 @@ task:
     - env: {BUILD: distcheck, WITH_VALGRIND: no, CTIMETESTS: no, BENCH: no}
     - env: {CPPFLAGS: -DDETERMINISTIC}
     - env: {CFLAGS: -O0, CTIMETESTS: no}
+    - env: {CFLAGS: -O1, RECOVERY: yes, ECDH: yes, SCHNORRSIG: yes, ELLSWIFT: yes}
     - env: { ECMULTGENPRECISION: 2, ECMULTWINDOW: 2 }
     - env: { ECMULTGENPRECISION: 8, ECMULTWINDOW: 4 }
   matrix:

src/ecmult_impl.h

@@ -288,7 +288,9 @@ static void secp256k1_ecmult_strauss_wnaf(const struct secp256k1_strauss_state *
     }
 
     /* Bring them to the same Z denominator. */
-    secp256k1_ge_table_set_globalz(ECMULT_TABLE_SIZE(WINDOW_A) * no, state->pre_a, state->aux);
+    if (no) {
+        secp256k1_ge_table_set_globalz(ECMULT_TABLE_SIZE(WINDOW_A) * no, state->pre_a, state->aux);
+    }
 
     for (np = 0; np < no; ++np) {
         for (i = 0; i < ECMULT_TABLE_SIZE(WINDOW_A); i++) {