vi: fix several instances of major goof: when text grows, text[]

vi: fix several instances of major goof: when text grows, text[]
 might get reallocated! We were keeping around pointers to old place...

function                                             old     new   delta
colon                                               3017    3037     +20
char_insert                                          336     354     +18
stupid_insert                                         18      24      +6
string_insert                                         89      94      +5
do_cmd                                              4461    4465      +4
file_insert                                          328     329      +1
text_hole_make                                       134     120     -14
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 6/1 up/down: 54/-14)             Total: 40 bytes