Fixed segmentation fault in case of much XML output

Because of the multi telegrams it is impossible to use a fixed
buffer for XML output on the stack. Now the buffer is allocated on the
heap. These also leads to a change in return behaviour for all XML
functions, because it's possible that the buffer is NULL. For variable
data this buffer grows every time there are less than 1024 bytes left
for a new record.

bin/mbus-serial-request-data-multi-reply.c

@@ -169,7 +169,9 @@ main(int argc, char **argv)
         fprintf(stderr, "Failed to generate XML representation of MBUS frames: %s\n", mbus_error_str());
         return 1;
     }
+    
     printf("%s", xml_result);
+    free(xml_result);
 
     mbus_disconnect(handle);
     return 0;