qemu: avoid crash on process attach

Detected by ccc-analyzer, reported by Alex Jia.

qemuProcessStart always calls qemuProcessWaitForMonitor with a
non-negative position, but qemuProcessAttach always calls with -1.
In the latter case, there is no log file we can scrape, so we
also should not be trying to scrape the logs if the qemu process
died at the very end.

* src/qemu/qemu_process.c (qemuProcessWaitForMonitor): Don't try
to read from log in qemuProcessAttach case.

src/qemu/qemu_process.c

@@ -1214,7 +1214,7 @@ qemuProcessWaitForMonitor(struct qemud_driver* driver,
 cleanup:
     virHashFree(paths);
 
-    if (kill(vm->pid, 0) == -1 && errno == ESRCH) {
+    if (pos != -1 && kill(vm->pid, 0) == -1 && errno == ESRCH) {
         /* VM is dead, any other error raised in the interim is probably
          * not as important as the qemu cmdline output */
         qemuProcessReadLogFD(logfd, buf, buf_size, strlen(buf));
@@ -1225,8 +1225,6 @@ cleanup:
     }
 
 closelog:
-    VIR_FREE(buf);
-
     if (VIR_CLOSE(logfd) < 0) {
         char ebuf[1024];
         VIR_WARN("Unable to close logfile: %s",