lib: use delete[] to free buffer allocated using new[]

Fix warning caught by clang:

lib/regexp-fields.cc:41:2: warning: 'delete' applied to a pointer that was allocated
      with 'new[]'; did you mean 'delete[]'? [-Wmismatched-new-delete]
        delete buffer;
        ^
              []
lib/regexp-fields.cc:37:17: note: allocated with 'new[]' here
        char *buffer = new char[len];
                       ^