[SSTV] Fixed issues found by cppcheck

src/protocols/SSTV/SSTV.cpp

@@ -246,7 +246,7 @@ void SSTVClient::sendHeader() {
   this->tone(RADIOLIB_SSTV_TONE_BREAK, RADIOLIB_SSTV_HEADER_BIT_LENGTH);
 }
 
-void SSTVClient::sendLine(uint32_t* imgLine) {
+void SSTVClient::sendLine(const uint32_t* imgLine) {
   // check first line flag in Scottie modes
   if(firstLine && ((txMode.visCode == RADIOLIB_SSTV_SCOTTIE_1) || (txMode.visCode == RADIOLIB_SSTV_SCOTTIE_2) || (txMode.visCode == RADIOLIB_SSTV_SCOTTIE_DX))) {
     firstLine = false;

src/protocols/SSTV/SSTV.h

@@ -174,7 +174,7 @@ class SSTVClient {
       \param imgLine Image line to send, in 24-bit RGB. It is up to the user to ensure that
       imgLine has enough pixels to send it in the current SSTV mode.
     */
-    void sendLine(uint32_t* imgLine);
+    void sendLine(const uint32_t* imgLine);
 
     /*!
       \brief Get picture height of the currently configured SSTV mode.