Fix cppcheck warnings in chain/*.cpp

[src/chain/block.cpp:82]: (performance) Variable 'result' is reassigned a value before the old one has been used.
[src/chain/input.cpp:84]: (performance) Variable 'result' is reassigned a value before the old one has been used.
[src/chain/operation.cpp:87]: (performance) Variable 'result' is reassigned a value before the old one has been used.
[src/chain/operation.cpp:95]: (style) The scope of the variable 'size' can be reduced.
[src/chain/output.cpp:81]: (performance) Variable 'result' is reassigned a value before the old one has been used.
[src/chain/point.cpp:84]: (performance) Variable 'result' is reassigned a value before the old one has been used.
[src/chain/transaction.cpp:87]: (performance) Variable result is reassigned a value before the old one has been used.

src/chain/block.cpp

@@ -77,9 +77,8 @@ bool block::from_data(std::istream& stream)
 
 bool block::from_data(reader& source)
 {
-    auto result = true;
     reset();
-    result = header.from_data(source, false);
+    auto result = header.from_data(source, false);
 
     if (result)
     {

src/chain/input.cpp

@@ -79,9 +79,8 @@ bool input::from_data(std::istream& stream)
 
 bool input::from_data(reader& source)
 {
-    auto result = true;
     reset();
-    result = previous_output.from_data(source);
+    auto result = previous_output.from_data(source);
 
     if (result)
     {

src/chain/operation.cpp

@@ -81,20 +81,19 @@ bool operation::from_data(std::istream& stream)
 
 bool operation::from_data(reader& source)
 {
-    auto result = true;
     reset();
     const auto byte = source.read_byte();
-    result = source;
+    auto result = static_cast<bool>(source);
 
     auto op_code = static_cast<opcode>(byte);
     if (byte == 0 && op_code != opcode::zero)
         return false;
 
     code = ((0 < byte && byte <= 75) ? opcode::special : op_code);
 
-    uint32_t size;
     if (operation::must_read_data(code))
     {
+        uint32_t size;
         read_opcode_data_size(size, code, byte, source);
         data = source.read_data(size);
         result = (source && (data.size() == size));

src/chain/output.cpp

@@ -75,10 +75,9 @@ bool output::from_data(std::istream& stream)
 
 bool output::from_data(reader& source)
 {
-    auto result = true;
     reset();
     value = source.read_8_bytes_little_endian();
-    result = source;
+    auto result = static_cast<bool>(source);
 
     if (result)
         result = script.from_data(source, true, 

src/chain/point.cpp

@@ -77,11 +77,10 @@ bool point::from_data(std::istream& stream)
 
 bool point::from_data(reader& source)
 {
-    auto result = true;
     reset();
     hash = source.read_hash();
     index = source.read_4_bytes_little_endian();
-    result = source;
+    auto result = static_cast<bool>(source);
     if (!result)
         reset();
 

src/chain/transaction.cpp

@@ -81,10 +81,9 @@ bool transaction::from_data(std::istream& stream)
 
 bool transaction::from_data(reader& source)
 {
-    auto result = true;
     reset();
     version = source.read_4_bytes_little_endian();
-    result = source;
+    auto result = static_cast<bool>(source);
 
     if (result)
     {