Fix double free issue

In commit e18eb69, retrieve_manifests() was refactored to support the
mixer integration feature, but it introduced a double free of the
"filename" pointer when following certain code paths.

One code path to reproduce the issue is when a Manifest.MoM is not
present in the state directory, and swupd_curl_check_network() fails.  A
free(filename) was being called immediately before
swupd_curl_check_network(), and then again after jumping to the "out"
label for the error condition.

Resolve the issue by resetting the filename pointer to NULL after
freeing the memory to prevent a later double free. I also reset the url
pointer to NULL at the second call site for similar reasoning.

Signed-off-by: Patrick McCarty <patrick.mccarty@intel.com>

src/manifest.c

@@ -544,6 +544,7 @@ static int retrieve_manifests(int current, int version, char *component, struct
 		goto out;
 	}
 	free(filename);
+	filename = NULL;
 
 	if (swupd_curl_check_network()) {
 		ret = -ENOSWUPDSERVER;
@@ -577,7 +578,9 @@ static int retrieve_manifests(int current, int version, char *component, struct
 			goto untar;
 		}
 		free(filename);
+		filename = NULL;
 		free(url);
+		url = NULL;
 	}
 
 	/* Either we're not on mix or it failed, try curl-ing the file if link didn't work */