Fix ASCII protocol bug when writing to uint8_t

When writing to uint8_t properties or equivalently
sized enum properties, the ASCII protocol handler would
write beyond the variable itself and overwrite adjacent
memory.

This manifested for instance in the following:

    r axis0.controller.config.input_mode 1
    w axis0.controller.config.control_mode 3
    r axis0.controller.config.input_mode 0

This boils down to what appears to be misbehavior of
`sscanf`.

This fix adds an intermediate union to provide a safe
memory area for the `sscanf` call.

Firmware/fibre/cpp/include/fibre/protocol.hpp

@@ -591,7 +591,17 @@ static bool to_string(const T& value, char * buffer, size_t length, ...) {
 
 template<typename T, typename = typename format_traits_t<T>::type>
 static bool from_string(const char * buffer, size_t length, T* property, int) {
-    return sscanf(buffer, format_traits_t<T>::fmt, property) == 1;
+    // Note for T == uint8_t: Even though we supposedly use the correct format
+    // string sscanf treats our pointer as pointer-to-int instead of
+    // pointer-to-uint8_t. To avoid an unexpected memory access we first read
+    // into a union.
+    union { T t; int i; } val;
+    if (sscanf(buffer, format_traits_t<T>::fmt, &val.t) == 1) {
+        *property = val.t;
+        return true;
+    } else {
+        return false;
+    }
 }
 // Special case for float because printf promotes float to double, and we get warnings
 template<typename T = float>