fix usage of non-existent attribute buffer

vncterm/VNConsole.c

@@ -171,6 +171,7 @@ void vcScroll(vncConsolePtr c,int lineCount)
 	    c->screenBuffer+(c->height-lineCount)*c->width,
 	    (c->height-lineCount)*c->width);
 #ifdef USE_ATTRIBUTE_BUFFER
+    if(c->attributeBuffer)
 	    memmove(c->attributeBuffer,
 		    c->attributeBuffer+(c->height-lineCount)*c->width,
 		    (c->height-lineCount)*c->width);
@@ -182,6 +183,7 @@ void vcScroll(vncConsolePtr c,int lineCount)
 	    c->screenBuffer,
 	    (c->height+lineCount)*c->width);
 #ifdef USE_ATTRIBUTE_BUFFER
+    if(c->attributeBuffer)
 	    memmove(c->attributeBuffer-lineCount*c->width,
 		    c->attributeBuffer,
 		    (c->height+lineCount)*c->width);
@@ -194,6 +196,7 @@ void vcScroll(vncConsolePtr c,int lineCount)
   memset(c->screenBuffer+y1/c->cHeight*c->width,' ',
 	 (y2-y1)/c->cHeight*c->width);
 #ifdef USE_ATTRIBUTE_BUFFER
+  if(c->attributeBuffer)
 	  memset(c->attributeBuffer+y1/c->cHeight*c->width,0x07,
 		 (y2-y1)/c->cHeight*c->width);
 #endif