remove the use of String's len field (introduced in #22133)

base/gmp.jl

@@ -586,18 +586,17 @@ hex(n::BigInt, pad::Int) = base(16, n, pad)
 function base(b::Integer, n::BigInt, pad::Integer=1)
  b < 0 && return base(Int(b), n, pad, (b>0) & (n.size<0))
  2 <= b <= 62 || throw(ArgumentError("base must be 2 ≤ base ≤ 62, got $b"))
+ iszero(n) && pad < 1 && return ""
  nd1 = ndigits(n, b)
  nd = max(nd1, pad)
- str = Base._string_n(nd + isneg(n) + 1) # +1 for final '\0'
- ptr = pointer(str)
+ sv = Base.StringVector(nd + isneg(n))
+ ptr = pointer(sv)
  MPZ.get_str!(ptr + nd - nd1, b, n)
  for i = (0:nd-nd1-1) + isneg(n)
  unsafe_store!(ptr+i, '0' % UInt8)
  end
  isneg(n) && unsafe_store!(ptr, '-' % UInt8)
- str.len -= 1 # final '\0'
- iszero(n) && pad < 1 && (str.len -= 1)
- str
+ String(sv)
 end
 
 function ndigits0zpb(x::BigInt, b::Integer)