rtl-sdr: Fix out-of-bounds tuner name lookup

Validate the tuner type index before accessing the tuner name table.
The previous check allowed an index equal to names.size(), which could
result in an out-of-range QStringList access.

Use a local integer value for the enum conversion and ensure only valid
indices are used when looking up tuner names.

pointed out by cppcheck: Access out of bounds

Signed-off-by: Robin Getz <rgetz503@gmail.com>

plugins/samplesource/rtlsdr/rtlsdrinput.cpp

@@ -469,8 +469,10 @@ QString RTLSDRInput::getTunerName() const
 {
     const static QStringList names = {"Unknown", "E4000", "FC0012", "FC0013", "FC2580", "R820T", "R828D"};
 
-    if ((int) m_tunerType <= names.size()) {
-        return names[(int) m_tunerType];
+    const int tunerType = (int) m_tunerType;
+
+    if (tunerType >= 0 && tunerType < names.size()) {
+        return names[tunerType];
     } else {
         return names[0];
     }