libbb: make xrealloc_vector zero out the realloc'ed tail

libbb: make xrealloc_vector zero out the realloc'ed tail

function                                             old     new   delta
xrealloc_vector_helper                                51      76     +25
man_main                                             712     705      -7
act                                                  250     234     -16
create_list                                           91      70     -21
getopt_main                                          695     664     -31
load_dep_bb                                          281     248     -33
fileAction                                           744     709     -35
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(add/remove: 0/0 grow/shrink: 1/6 up/down: 25/-143)          Total: -118 bytes