Using new rather than malloc in debug_print

Using malloc will require us to add stdlib.h somewhere in the path for
the application. Maybe the CI apps are adding stdlib.h and that's why
the code would have worked. In a custom app, it can happen that the
header is not included. Using new avoids the need to add stdlib.h
anywhere and it is more in line with C++.

features/cellular/framework/AT/ATHandler.cpp

@@ -1289,7 +1289,7 @@ void ATHandler::debug_print(const char *p, int len, ATType type)
 #if MBED_CONF_CELLULAR_DEBUG_AT
     if (_debug_on) {
         const int buf_size = len * 4 + 1; // x4 -> reserve space for extra characters, +1 -> terminating null
-        char *buffer = (char *)malloc(buf_size);
+        char *buffer = new char [buf_size];
         if (buffer) {
             memset(buffer, 0, buf_size);
 
@@ -1319,7 +1319,7 @@ void ATHandler::debug_print(const char *p, int len, ATType type)
                 tr_info("AT ERR (%2d): %s", len, buffer);
             }
 
-            free(buffer);
+            delete [] buffer;
         } else {
             tr_error("AT trace unable to allocate buffer!");
         }