qemuDomainAttach: Initialize pidfile variable

If parsing qemu command line fails (e.g. because of non-existing
process number supplied), we jump to cleanup label where we free
pidfile. Therefore it needs to be initialized. Otherwise we free
random pointer.

src/qemu/qemu_driver.c

@@ -10179,7 +10179,7 @@ static virDomainPtr qemuDomainAttach(virConnectPtr conn,
     virDomainPtr dom = NULL;
     virDomainChrSourceDefPtr monConfig = NULL;
     bool monJSON = false;
-    char *pidfile;
+    char *pidfile = NULL;
 
     virCheckFlags(0, NULL);