modbus: fix compiler uninitialized warnings with -Wmaybe-uninitialized

All variables are initialized thanks to ModbusExtractUint8 or ModbusExtractUint16
function that extracts 8bits or 16bits data from pointer the received input data.
In case of extracting error (because of length), ModbusExtractUint8 or
ModbusExtractUint16 returns an error that is managed by the caller function.

All variables are now initialized to zero when they are declared. It does not
change anything functionnally but it removes Modbus warnings.

src/app-layer-modbus.c

@@ -490,7 +490,7 @@ static void ModbusExceptionResponse(ModbusTransaction   *tx,
                                     uint16_t            *offset)
 {
     SCEnter();
-    uint8_t exception;
+    uint8_t exception = 0;
 
     /* Exception code (1 byte) */
     if (ModbusExtractUint8(modbus, &exception, input, input_len, offset))
@@ -596,7 +596,7 @@ static void ModbusParseReadResponse(ModbusTransaction   *tx,
                                     uint16_t            *offset)
 {
     SCEnter();
-    uint8_t count;
+    uint8_t count = 0;
 
     /* Count (1 bytes) */
     if (ModbusExtractUint8(modbus, &count, input, input_len, offset))
@@ -646,8 +646,8 @@ static int ModbusParseWriteRequest(ModbusTransaction   *tx,
                                    uint16_t            *offset)
 {
     SCEnter();
-    uint16_t    quantity = 1, word;
-    uint8_t     byte, count = 1, type = tx->type;
+    uint16_t    quantity = 1, word = 0;
+    uint8_t     byte = 0, count = 1, type = tx->type;
 
     int i = 0;
 
@@ -773,7 +773,7 @@ static void ModbusParseWriteResponse(ModbusTransaction   *tx,
                                      uint16_t            *offset)
 {
     SCEnter();
-    uint16_t    address, quantity, word;
+    uint16_t    address = 0, quantity = 0, word = 0;
     uint8_t     type = tx->type;
 
     /* Starting Address (2 bytes) */
@@ -863,7 +863,7 @@ static int ModbusParseDiagnosticRequest(ModbusTransaction   *tx,
                                         uint16_t            *offset)
 {
     SCEnter();
-    uint16_t data;
+    uint16_t data = 0;
 
     /* Sub-function (2 bytes) */
     if (ModbusExtractUint16(modbus, &(tx->subFunction), input, input_len, offset))
@@ -961,7 +961,7 @@ static void ModbusParseRequestPDU(ModbusTransaction *tx,
 {
     SCEnter();
     uint16_t    offset = (uint16_t) sizeof(ModbusHeader);
-    uint8_t     count;
+    uint8_t     count = 0;
 
     int i = 0;
 
@@ -1118,7 +1118,7 @@ static void ModbusParseResponsePDU(ModbusTransaction    *tx,
 {
     SCEnter();
     uint16_t    offset = (uint16_t) sizeof(ModbusHeader);
-    uint8_t     count, error = FALSE, function, mei;
+    uint8_t     count = 0, error = FALSE, function = 0, mei = 0;
 
     /* Standard function codes used on MODBUS application layer protocol (1 byte) */
     if (ModbusExtractUint8(modbus, &function, input, input_len, &offset))