[libc++] Wraparound in string `operator>>` causes issues with `-fsanitize=integer`

[ldionne]

Consider the following program:

#include <string>
#include <fstream>
#include <iostream>

int main() {
    std::string foo;
    std::ifstream fin("whatever");
    fin >> foo;
    std::cout << "read " << foo << std::endl;
}

With -fsanitize=integer, we get:

include/c++/v1/istream:1213:15: runtime error: implicit conversion from type 'size_type' (aka 'unsigned long') of value 18446744073709551607 (64-bit, unsigned) to type 'streamsize' (aka 'long') changed the value to -9 (64-bit, signed)

This is because the code does this:

__str.clear();
streamsize __n = __is.width();
if (__n <= 0)
  __n = __str.max_size();
if (__n <= 0)
  __n = numeric_limits<streamsize>::max();

So basically -fsanitize=integer is complaining about the wraparound.
Although we're handling the wraparound afterwards with the second if, it would be easy to write this code in a less confusing way.