lib/itoa: fix array subscript below bounds warning

[haubi]

Trivial fix for this warning seen with gcc-8.2:

itoa.cc: In function 'const char* itoa(long int, int)':
itoa.cc:18:32: warning: array subscript -1 is below array bounds of 'char [64]' [-Warray-bounds]
   while(digits > 0 && ptr > buf-1)
                             ~~~^~