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.

changes/bug10409

@@ -0,0 +1,3 @@
+  o Minor bugfixes:
+    - Avoid a crash bug when starting with a corrupted microdescriptor
+      cache file. Fix for bug 10406; bugfix on 0.2.2.6-alpha.