Fix build error with Werror=sign-compare is enabled (#283)

<!--- Title -->

Description
-----------
This PR fixes a build issue when `Werror=sign-compare` is enabled while
building the library.

Test Steps
-----------
<!-- Describe the steps to reproduce. -->

Checklist:
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- [x] I have tested my changes. No regression in existing tests.
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in this Pull Request.~

Related Issue
-----------
#282 

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.github/workflows/ci.yml

@@ -32,7 +32,7 @@ jobs:
           -DCMAKE_BUILD_TYPE=Debug \
           -DBUILD_CLONE_SUBMODULES=ON \
           -DUNITTEST=1 \
-          -DCMAKE_C_FLAGS='--coverage -Wall -Wextra -Werror -DNDEBUG -DLIBRARY_LOG_LEVEL=LOG_DEBUG'
+          -DCMAKE_C_FLAGS='--coverage -Wall -Wextra -Wsign-compare -Werror -DNDEBUG -DLIBRARY_LOG_LEVEL=LOG_DEBUG'
           make -C build/ all
           echo "::endgroup::"
 

source/core_mqtt.c

@@ -2217,7 +2217,7 @@ static MQTTStatus_t sendConnectWithoutCopy( MQTTContext_t * pContext,
                                                    pWillInfo,
                                                    remainingLength );
 
-        assert( ( pIndex - connectPacketHeader ) <= sizeof( connectPacketHeader ) );
+        assert( ( ( size_t ) ( pIndex - connectPacketHeader ) ) <= sizeof( connectPacketHeader ) );
 
         /* The header gets sent first. */
         iterator->iov_base = connectPacketHeader;