isprint() wants value in range [-1, 255]

The MSVC impl. assert()s this

src/factory/printer.cpp

@@ -497,7 +497,7 @@ std::ostream& operator<<(std::ostream& strm, const escape& Q)
         case '\'': next = '\''; break;
         case '\"': next = '\"'; if(Q.S==escape::CSV) quote = '"'; break;
         default:
-            if(!isprint(C)) {
+            if(!isprint((unsigned char)C)) {
                 // print three charator escape
                 strm<<"\\x"<<hexdigit(C>>4)<<hexdigit(C);
             } else {
@@ -534,7 +534,7 @@ std::ostream& operator<<(std::ostream& strm, const maybeQuote& q)
             esc = true;
             break;
         default:
-            if(!isprint(q.s[i])) {
+            if(!isprint((unsigned char)q.s[i])) {
                 esc = true;
             }
             break;