Runtime Error in MQTTPacket_formatPayload Due to char Type Not Defaulting to Unsigned in MSVC 193833134 (Visual Studio 2022)

[liuyuanlins]

Issue Description:

Compilation Environment: Visual Studio 2022
Compiler Version: MSVC 193833134

In the file paho.mqtt.c\src\MQTTPacket.c, the function:

int MQTTPacket_formatPayload(int buflen, char* buf, int payloadlen, char* payload)

at line 863:

if (isprint(payload[i]))

the isprint causes an error during debugging with the isprint function. This is because the compiler does not default the char type to unsigned, as shown in the attached image.

convert isclype.cpp Line: 36 Expression:c>=-1&&c<= 255