growpart fails if size of partition == start of partition

If the size of a partition is equal to the start of a partition, growpart will fail to correctly grow the partition (and will probably end up breaking things). For example, we start with the following partition layout:

    [root@growpart ~]# sfdisk --unit=S --dump /dev/vdb
    # partition table of /dev/vdb
    unit: sectors

    /dev/vdb1 : start= 2048, size= 1024000, Id=83
    /dev/vdb2 : start= 1026048, size= 1026048, Id=83
    /dev/vdb3 : start= 0, size= 0, Id= 0
    /dev/vdb4 : start= 0, size= 0, Id= 0

Growpart modifies that table with the following sed expression:

    sed "\|^\s*${dpart} |s/${pt_size},/${new_size},/" "${dump_out}" \
            >"${new_out}" ||
            fail "failed to change size in output"

That becomes:

    sed '\|^\s*/dev/vdb2 |s/1026048,/3168223,/' /tmp/growpart.XuXMSx/dump.out

That will erroneously match the partition start, rather than the
partition size, which results in the partition map looking like:

    [root@growpart ~]# sfdisk --unit=S --dump /dev/vdb
    # partition table of /dev/vdb
    unit: sectors

    /dev/vdb1 : start= 2048, size= 1024000, Id=83
    /dev/vdb2 : start= 3168223, size= 1026048, Id=83
    /dev/vdb3 : start= 0, size= 0, Id= 0
    /dev/vdb4 : start= 0, size= 0, Id= 0

Notice that the partition is still the same size, but it now starts at
the wrong location.