Fix undefined left shift in SimpleDeserializer::readS64

The signed 64-bit deserialization code used a signed temporary value while
assembling bytes:

    tmp = (tmp << 8) | byte;

For negative values, tmp was initialized to -1 for sign extension. Left-shifting
this negative value is undefined behavior in C++.

Use an unsigned temporary while constructing the 64-bit representation and
convert to qint64 after all bytes have been assembled. This preserves the
existing two's complement sign handling while avoiding undefined behavior.

Fixes static analysis warning about shifting a negative value.

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

sdrbase/util/simpleserializer.cpp

@@ -392,7 +392,7 @@ returnDefault:
 bool SimpleDeserializer::readS64(quint32 id, qint64* result, qint64 def) const
 {
 	uint readOfs;
-	qint64 tmp;
+	quint64 tmp;
 	Elements::const_iterator it = m_elements.constFind(id);
 	if(it == m_elements.constEnd())
 		goto returnDefault;
@@ -406,10 +406,10 @@ bool SimpleDeserializer::readS64(quint32 id, qint64* result, qint64 def) const
 	for(uint i = 0; i < it->length; i++) {
 		quint8 byte = readByte(&readOfs);
 		if((i == 0) && (byte & 0x80))
-			tmp = -1;
+			tmp = 0xFFFFFFFFFFFFFFFFULL;
 		tmp = (tmp << 8) | byte;
 	}
-	*result = tmp;
+	*result = static_cast<qint64>(tmp);
 	return true;
 
 returnDefault: