imv-scanner: Fix potential buffer overflow

While `pos` was moved to the end, `len` was not adjusted (i.e. set to 0)
so later calls could write beyond the buffer.  However, the last port
written might have been incomplete, so instead we just reset the string.

src/libimcv/plugins/imv_scanner/imv_scanner_agent.c

@@ -429,7 +429,6 @@ METHOD(imv_agent_if_t, batch_ending, TNC_Result,
 					written = snprintf(pos, len, " %u", port);
 					if (written < 0 || written >= len)
 					{
-						pos += len - 1;
 						*pos = '\0';
 					}
 					else