I claim the ornamental design for a bottle preform, as shown and described.
FIG. 1 is a rear elevation view of a bottle preform according to our new design;
FIG. 2 is a left side elevation view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a top, right side perspective view thereof.
The evenly-dashed broken lines in the drawings illustrate the portions of the bottle preform that form no part of the claim. The dot-dash broken lines in the drawings define the bounds of the claim and form no part thereof.