fix return type of check MeSuperVariable::set_variable

[s-urbaniak]

This fixes the following compilation error:

/home/sur/Arduino/libraries/makeblock/src/MeSuperVariable.cpp: In member function 'void* MeSuperVariable::set_variable(char*, uint8_t, void*, uint8_t)':
/home/sur/Arduino/libraries/makeblock/src/MeSuperVariable.cpp:241:14: error: cannot convert 'bool' to 'void*' in return
       return false;
              ^~~~~

[nbourre]

@Makeblock-official : This simple modification fix the compilation problems!