Fix some Infer's warnings #35732

Fix DEAD_STORE warnings found by the Infer static analyzer tool:

src/policy/fees/block_policy_estimator.cpp:881: error: Dead Store(DEAD_STORE)
  The value written to `&median` is never used.
  879.     }
  880.
  881.     double median = -1;
           ^
  882.     EstimationResult tempResult;
  883.

src/test/scriptnum_tests.cpp:58: error: Dead Store(DEAD_STORE)
  The value written to `&bignum3` is never used.
  56.     const CScriptNum scriptnum1(num1);
  57.     const CScriptNum scriptnum2(num2);
  58.     CScriptNum10 bignum3(num1);
                       ^
  59.     CScriptNum10 bignum4(num1);
  60.     CScriptNum scriptnum3(num1);

src/test/scriptnum_tests.cpp:59: error: Dead Store(DEAD_STORE)
  The value written to `&bignum4` is never used.
  57.     const CScriptNum scriptnum2(num2);
  58.     CScriptNum10 bignum3(num1);
  59.     CScriptNum10 bignum4(num1);
                       ^
  60.     CScriptNum scriptnum3(num1);
  61.     CScriptNum scriptnum4(num1);

src/test/scriptnum_tests.cpp:60: error: Dead Store(DEAD_STORE)
  The value written to `&scriptnum3` is never used.
  58.     CScriptNum10 bignum3(num1);
  59.     CScriptNum10 bignum4(num1);
  60.     CScriptNum scriptnum3(num1);
                     ^
  61.     CScriptNum scriptnum4(num1);
  62.

src/test/scriptnum_tests.cpp:61: error: Dead Store(DEAD_STORE)
  The value written to `&scriptnum4` is never used.
  59.     CScriptNum10 bignum4(num1);
  60.     CScriptNum scriptnum3(num1);
  61.     CScriptNum scriptnum4(num1);
                     ^
  62.
  63.     // int64_t overflow is undefined.

src/util/subprocess.h:1263: error: Dead Store(DEAD_STORE)
  The value written to `&sys_ret` is never used.
  1261. #ifndef WIN32
  1262.   inline void Child::execute_child() {
  1263.     int sys_ret = -1;
            ^
  1264.     auto& stream = parent_->stream_;
  1265.

src/validation.cpp:997: error: Dead Store(DEAD_STORE)
  The value written to `&newFeeRate` is never used.
  995.     TxValidationState& state = ws.m_state;
  996.
  997.     CFeeRate newFeeRate(ws.m_modified_fees, ws.m_vsize);
                    ^
  998.
  999.     CTxMemPool::setEntries all_conflicts;

src/wallet/migrate.cpp:538: error: Dead Store(DEAD_STORE)
  The value written to `&page_size` is never used.
  536.     }
  537.
  538.     uint32_t page_size = 4096; // Default page size
           ^
  539.
  540.     // Read the outer metapage

src/util/subprocess.h:488: error: Dead Store(DEAD_STORE)
  The value written to `&fill_sz` is never used.
  486.       } else { // Partial data ? Continue reading
  487.         total_bytes_read += rd_bytes;
  488.         fill_sz -= rd_bytes;
               ^
  489.         break;
  490.       }

Fix a NULLPTR_DEREFERENCE warning:

src/wallet/external_signer_scriptpubkeyman.cpp:71: error: Null Dereference(NULLPTR_DEREFERENCE)
  `provider.__infer_backing_pointer` could be null (from the call to `wallet::DescriptorScriptPubKeyMan::GetSolvingProvider()` on line 70) and is dereferenced.
  69.     const CScript& scriptPubKey = GetScriptForDestination(dest);
  70.     auto provider = GetSolvingProvider(scriptPubKey);
  71.     auto descriptor = InferDescriptor(scriptPubKey, *provider);
                                                          ^
  72.
  73.     const UniValue& result = signer.DisplayAddress(descriptor->ToString());