A field in format B or beyond is not expected to re-read its prefix stored
externally when it has already stored the prefix in memory. Or else the result
would be local prefix appended by external prefix which must be wrong.

Solution:
In row_ext_cache_fill where the external stored value will be read, if the
file format is barracuda or beyond and the prefix is already in memory,
do a memcpy to copy the in memory prefix to ext.

rb#2921 Approved by Jimmy