The ornamental design for the refrigerator piggy bank, as shown and described.
FIG. 1 is a top front perspective view a refrigerator piggy bank of my new design;
FIG. 2 is a left side elevation view thereof;
FIG. 3 is a top rear perspective view thereof, with an access door disengaged;
FIG. 4 is a bottom rear perspective view thereof; and,
FIG. 5 is a right side elevation view thereof.
The broken line showing of a refrigerator in FIG. 1 and a coin in FIG. 3 are included for the purpose of illustrating environmental structure and forms no part of the claimed design.