We claim the ornamental design for an inflatable mattress, as shown and described.
FIG. 1 is a perspective view of a first inflatable mattress, where a body of the first inflatable mattress is shown in broken lines;
FIG. 2 is a top plan view of the first inflatable mattress of FIG. 1;
FIG. 3 is a bottom plan view of the first inflatable mattress of FIG. 1;
FIG. 4 is a side elevational view of the first inflatable mattress of FIG. 1;
FIG. 5 is an end elevational view of the first inflatable mattress of FIG. 1;
FIG. 6 is a perspective view of a second inflatable mattress different in size than the first inflatable mattress of FIG. 1, where a body of the second inflatable mattress is shown in broken lines;
FIG. 7 is a top plan view of the second inflatable mattress of FIG. 6;
FIG. 8 is a bottom plan view of the second inflatable mattress of FIG. 6;
FIG. 9 is a side elevational view of the second inflatable mattress of FIG. 6; and,
FIG. 10 is an end elevational view of the second inflatable mattress of FIG. 6.
The broken lines in FIGS. 1-10 represent unclaimed portions of the inflatable mattress and form no part of the claimed design.