Avoid free()ing from an mmap on corrupted microdesc cache

The 'body' field of a microdesc_t holds a strdup()'d value if the
microdesc's saved_location field is SAVED_IN_JOURNAL or
SAVED_NOWHERE, and holds a pointer to the middle of an mmap if the
microdesc is SAVED_IN_CACHE.  But we weren't setting that field
until a while after we parsed the microdescriptor, which left an
interval where microdesc_free() would try to free() the middle of
the mmap().

This patch also includes a regression test.

This is a fix for #10409; bugfix on 0.2.2.6-alpha.

src/or/routerparse.h

@@ -64,7 +64,7 @@ ns_detached_signatures_t *networkstatus_parse_detached_signatures(
 
 smartlist_t *microdescs_parse_from_string(const char *s, const char *eos,
                                           int allow_annotations,
-                                          int copy_body);
+                                          saved_location_t where);
 
 authority_cert_t *authority_cert_parse_from_string(const char *s,
                                                    const char **end_of_string);