Fix off-by-one error in BRIGHT and TERRAIN lump parsing

Fixes a character or new-line being required after the last definition in TERRAIN and BRIGHT lumps, a common footgun as other lump types do not require this.

Test WAD: JUG_TerrainTest_EOFEndsDefinition.pk3

Test Steps:

• Load test wad: addfile JUG_TerrainTest_EOFEndsDefinition.pk3
• Load level: map JUG_TERRAINTEST
• The light gray-colored Stairs texture should have its brightmap and terrain type (SpecialStageWood) set.

Before (v2.3):

After: