smartlist_len cast to double

When building with DEBUG_SMARTLIST:

src/or/routerlist.c:2767:45: warning: cast from function call of type 'int' to non-matching type 'double' [-Wbad-function-cast]
    return ((double)n_with_descs) / (double)smartlist_len(sl);
                                            ^~~~~~~~~~~~~~~~~
1 warning generated.

src/or/routerlist.c:2767:45: warning: cast from function call of type 'int' to non-matching type 'double' [-Wbad-function-cast]
    return ((double)n_with_descs) / (double)smartlist_len(sl);
                                            ^~~~~~~~~~~~~~~~~
1 warning generated.

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