virConnectAuthCallbackDefault: Return failure if 'virGetPassword' returns NULL

virGetPassword can return NULL on linux or BSD if it fails. The caller
in virConnectAuthCallbackDefault does dereference it unconditionally.

Return failure if virGetPassword returns NULL.

Fixes: db72866310
Closes: https://gitlab.com/libvirt/libvirt/-/issues/777
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>

src/libvirt.c

@@ -158,7 +158,9 @@ virConnectAuthCallbackDefault(virConnectCredentialPtr cred,
             if (fflush(stdout) != 0)
                 return -1;
 
-            bufptr = virGetPassword();
+            if (!(bufptr = virGetPassword()))
+                return -1;
+
             if (STREQ(bufptr, ""))
                 VIR_FREE(bufptr);
             break;