fix: warning: adding 'const char' to a string does not append to the …

[shogo82148]

…string [-Wstring-plus-int]

https://github.com/shogo82148/mecab/runs/431774760?check_suite_focus=true#step:4:295

writer.cpp:255:61: warning: adding 'const char' to a string does not append to the string [-Wstring-plus-int]
            const std::string error = "unknown meta char: " + *p;
                                      ~~~~~~~~~~~~~~~~~~~~~~^~~~
writer.cpp:255:61: note: use array indexing to silence this warning
            const std::string error = "unknown meta char: " + *p;
                                                            ^
                                      &                     [   ]

[shogo82148]

before:

$ mecab --node-format=%z
ほげ
libc++abi.dylib: terminating with uncaught exception of type std::bad_alloc: std::bad_alloc
Abort trap: 6

after:

$ ./src/mecab --node-format=%z
ほげ
unknown meta char: z