Fix crash on image without any colors (all transparent)

src/gfx/pal_packing.cpp

@@ -279,6 +279,8 @@ static void verboseOutputAssignments(
 // LCOV_EXCL_STOP
 
 static void decant(std::vector<AssignedSets> &assignments, std::vector<ColorSet> const &colorSets) {
+	assume(!assignments.empty());
+
 	// "Decanting" is the process of moving all *things* that can fit in a lower index there
 	auto decantOn = [&assignments](auto const &tryDecanting) {
 		// No need to attempt decanting on palette #0, as there are no palettes to decant to

src/gfx/process.cpp

@@ -377,6 +377,7 @@ static std::pair<std::vector<size_t>, std::vector<Palette>>
 			pal.addColor(color);
 		}
 	}
+	assume(!palettes.empty());
 
 	// "Sort" colors in the generated palettes, see the man page for the flowchart
 	if (options.palSpecType == Options::DMG) {
@@ -1150,6 +1151,10 @@ continue_visiting_tiles:;
 	}
 	// LCOV_EXCL_STOP
 
+	if (colorSets.empty()) {
+		fatal("Image does not contain any colors");
+	}
+
 	if (options.palSpecType == Options::EMBEDDED) {
 		generatePalSpec(image);
 	}

test/gfx/no_colors.err

@@ -0,0 +1,2 @@
+FATAL: Image does not contain any colors
+Conversion aborted after 1 error