Merge #11332: Fix possible crash with invalid nCustomFeeRadio in QSettings (achow101, TheBlueMatt)

cdaf3a1 Fix Qt 0.14.2->0.15.0 segfault if "total at least" is selected (Matt Corallo)

Pull request description:

  `QButtonGroup->button()` may return a nullptr.
  Accessing the object directly with `setChecked` seems fragile.

  This is a simple fix to ensure to never call a button out of bounds (nullptr).

  There are probably other places where a sanity check for `QSettings` are required.

  Found by @achow101.
  Code by @TheBlueMatt.

Tree-SHA512: a1b5d6636382a4e20c4e66ef82de19e6daa8b1b5f21b0f2bc5f51cfb6b37797045c7e29ebead8088ee2b990ed12c549c217cae6aad7566319599d086d526f6dc

src/qt/sendcoinsdialog.cpp

@@ -128,7 +128,7 @@ SendCoinsDialog::SendCoinsDialog(const PlatformStyle *_platformStyle, QWidget *p
     ui->groupFee->setId(ui->radioCustomFee, 1);
     ui->groupFee->button((int)std::max(0, std::min(1, settings.value("nFeeRadio").toInt())))->setChecked(true);
     ui->groupCustomFee->setId(ui->radioCustomPerKilobyte, 0);
-    ui->groupCustomFee->button((int)std::max(0, std::min(1, settings.value("nCustomFeeRadio").toInt())))->setChecked(true);
+    ui->groupCustomFee->button(0)->setChecked(true);
     ui->customFee->setValue(settings.value("nTransactionFee").toLongLong());
     ui->checkBoxMinimumFee->setChecked(settings.value("fPayOnlyMinFee").toBool());
     minimizeFeeSection(settings.value("fFeeSectionMinimized").toBool());