fix memory leak in vector allocation

fix memory leak in vector allocation

Address Sanitizer detected a few instances of the following:

  Direct leak of 168 byte(s) in 7 object(s) allocated from:
    #0 0x7f4bc2724330 in __interceptor_malloc (/lib/x86_64-linux-gnu/libasan.so.5+0xe9330)
    #1 0x7f4bc24b1ea3 in vec_new /tmp/tmp.m68Z0dFAYs/graphviz/lib/common/routespl.c:918
    #2 0x7f4bc24b291a in find_all_cycles /tmp/tmp.m68Z0dFAYs/graphviz/lib/common/routespl.c:1073
    #3 0x7f4bc24b2b64 in get_cycle_centroid /tmp/tmp.m68Z0dFAYs/graphviz/lib/common/routespl.c:1121
    #4 0x7f4bc24b3c5c in makeStraightEdges /tmp/tmp.m68Z0dFAYs/graphviz/lib/common/routespl.c:1233
    #5 0x7f4bc24b35f8 in makeStraightEdge /tmp/tmp.m68Z0dFAYs/graphviz/lib/common/routespl.c:1203
    #6 0x7f4bbdf28b52 in _spline_edges /tmp/tmp.m68Z0dFAYs/graphviz/lib/neatogen/neatosplines.c:659
    #7 0x7f4bbdf29240 in splineEdges /tmp/tmp.m68Z0dFAYs/graphviz/lib/neatogen/neatosplines.c:736
    #8 0x7f4bbdf29310 in spline_edges1 /tmp/tmp.m68Z0dFAYs/graphviz/lib/neatogen/neatosplines.c:749
    #9 0x7f4bbdf293be in spline_edges0 /tmp/tmp.m68Z0dFAYs/graphviz/lib/neatogen/neatosplines.c:779
    #10 0x7f4bbdf29bcb in spline_edges /tmp/tmp.m68Z0dFAYs/graphviz/lib/neatogen/neatosplines.c:817
    #11 0x7f4bbdf14ea6 in neato_layout /tmp/tmp.m68Z0dFAYs/graphviz/lib/neatogen/neatoinit.c:1489
    #12 0x7f4bc242b6fc in gvLayoutJobs /tmp/tmp.m68Z0dFAYs/graphviz/lib/gvc/gvlayout.c:85
    #13 0x559fcd474960 in main /tmp/tmp.m68Z0dFAYs/graphviz/cmd/dot/dot.c:132
    #14 0x7f4bc21d709a in __libc_start_main ../csu/libc-start.c:308

find_all_cycles() was calling vec_delete() on the temporary vector it allocated,
but it did not take into account that the nested vectors were not freed by
vec_delete(). This change moves the open coded clean up logic from
get_cycle_centroid() into vec_delete() to fix this and make it less likely that
this is missed in future.

This was initially encountered while investigating #1554.