src: squelch -Wmaybe-uninitialized warning

The variable isn't actually used uninitialized but g++ 4.8 doesn't know
that.  Set it to NULL to silence the following compiler warning:

    ../src/string_bytes.cc:247:29: warning: 'data' may be used
    uninitialized in this function [-Wmaybe-uninitialized]
         unsigned a = hex2bin(src[i * 2 + 0]);
                                  ^
    ../src/string_bytes.cc:299:15: note: 'data' was declared here
       const char* data;
                   ^