Avoid using pointer in Ryu (#59743)

Author: nhz2

base/ryu/shortest.jl

@@ -378,8 +378,11 @@ function writeshortest(buf::AbstractVector{UInt8}, pos, x::T,
         else
             pointoff = olength - abs(nexp)
             # shift bytes after pointoff to make room for decchar
-            ptr = pointer(buf)
-            memmove(ptr + pos + pointoff, ptr + pos + pointoff - 1, olength - pointoff + 1)
+            buf_cconv = Base.cconvert(Ptr{UInt8}, buf)
+            GC.@preserve buf_cconv begin
+                ptr = Base.unsafe_convert(Ptr{UInt8}, buf_cconv)
+                memmove(ptr + pos + pointoff, ptr + pos + pointoff - 1, (olength - pointoff + 1)%Csize_t)
+            end
             @inbounds buf[pos + pointoff] = decchar
             pos += olength + 1
             precision -= olength