str2wcs: encode invalid Unicode characters in the private use area

Rust does not like invalid code points, so let's ease the transition by
treating them like byte sequences that do not map to any code point.
See fish-shell#9688 (comment)

Author: krobelus

src/common.cpp

@@ -338,6 +338,8 @@ static wcstring str2wcs_internal(const char *in, const size_t in_len) {
             // Determine whether to encode this character with our crazy scheme.
             if (wc >= ENCODE_DIRECT_BASE && wc < ENCODE_DIRECT_BASE + 256) {
                 use_encode_direct = true;
+            } else if ((wc >= 0xD800 && wc <= 0xDFFF) || static_cast<uint32_t>(wc) >= 0x110000) {
+                use_encode_direct = true;
             } else if (wc == INTERNAL_SEPARATOR) {
                 use_encode_direct = true;
             } else if (ret == static_cast<size_t>(-2)) {