Restore fullname[] contents before falling through in pg_open_tzfile().

Fix oversight in commit af2c5aa88: if the shortcut open() doesn't work,
we need to reset fullname[] to be just the name of the toplevel tzdata
directory before we fall through into the pre-existing code.  This failed
to be exposed in my (tgl's) testing because the fall-through path is
actually never taken under normal circumstances.

David Rowley, per report from Amit Kapila

Discussion: https://postgr.es/m/CAA4eK1LC7CaNhRAQ__C3ht1JVrPzaAXXhEJRnR5L6bfYHiLmWw@mail.gmail.com

src/timezone/pgtz.c

@@ -105,6 +105,7 @@ pg_open_tzfile(const char *name, char *canonname)
 		if (result >= 0)
 			return result;
 		/* If that didn't work, fall through to do it the hard way */
+		fullname[fullnamelen] = '\0';
 	}
 
 	/*