Tidy up Screen:fb2bmp() function

1. Remove unused assert() around io.open of the input device. The failure
to open input device is already guarded by the "if inputf" code.
2. Remove unneeded assert() around io.open of the output device. This is
unneeded because we should not crash the whole application just because
we cannot write screen dump (e.g. because filesystem is full, etc) ---
rather we should exit gracefully.

screen.lua

@@ -146,41 +146,43 @@ a vertical flip that makes it a bit slower, namely,
 NB: needs free memory of G_width*G_height/2 bytes to manupulate the fb-content! ]]
 
 function Screen:fb2bmp(fin, fout, vflip, pack) -- atm, for 4bpp framebuffers only
-	local inputf = assert(io.open(fin,"rb"))
+	local inputf = io.open(fin,"rb")
 	if inputf then
-		local outputf, size = assert(io.open(fout,"wb"))
-		-- writing bmp-header
-		outputf:write(string.char(0x42,0x4D,0xF6,0xA9,3,0,0,0,0,0,0x76,0,0,0,40,0),
-				self:LE(G_width), self:LE(G_height),	-- width & height: 4 chars each
-				string.char(0,0,1,0,4,0,0,0,0,0),
-				self:LE(G_height*G_width/2),		-- raw bytes in image
-				string.char(0x87,0x19,0,0,0x87,0x19,0,0),	-- 6536 pixel/m = 166 dpi for both x&y resolutions
-				string.char(16,0,0,0,0,0,0,0))		-- 16 colors
-		local line, i = G_width/2, 15
-		-- add palette to bmp-header
-		while i>=0 do
-			outputf:write(string.char(i*16+i):rep(3), string.char(0))
-			i=i-1
-		end
-		if vflip then -- flip image vertically to make it bmp-compliant
-			-- read the fb-content line-by-line & fill the content-table in the inversed order
-			local content = {}
-			for i=1, G_height do
-				table.insert(content, 1, inputf:read(line))
+		local outputf, size = io.open(fout,"wb")
+		if outputf then
+			-- writing bmp-header
+			outputf:write(string.char(0x42,0x4D,0xF6,0xA9,3,0,0,0,0,0,0x76,0,0,0,40,0),
+					self:LE(G_width), self:LE(G_height),	-- width & height: 4 chars each
+					string.char(0,0,1,0,4,0,0,0,0,0),
+					self:LE(G_height*G_width/2),		-- raw bytes in image
+					string.char(0x87,0x19,0,0,0x87,0x19,0,0),	-- 6536 pixel/m = 166 dpi for both x&y resolutions
+					string.char(16,0,0,0,0,0,0,0))		-- 16 colors
+			local line, i = G_width/2, 15
+			-- add palette to bmp-header
+			while i>=0 do
+				outputf:write(string.char(i*16+i):rep(3), string.char(0))
+				i=i-1
 			end
-			-- write the v-flipped bmp-data
-			for i=1, G_height do
-				outputf:write(content[i])
+			if vflip then -- flip image vertically to make it bmp-compliant
+				-- read the fb-content line-by-line & fill the content-table in the inversed order
+				local content = {}
+				for i=1, G_height do
+					table.insert(content, 1, inputf:read(line))
+				end
+				-- write the v-flipped bmp-data
+				for i=1, G_height do
+					outputf:write(content[i])
+				end
+			else -- without v-flip, it takes only 0.02s @ 600x800, 4bpp
+				outputf:write(inputf:read("*all"))
 			end
-		else -- without v-flip, it takes only 0.02s @ 600x800, 4bpp
-			outputf:write(inputf:read("*all"))
-		end
+			outputf:close()
+			-- here one may use either standard archivers (bzip2, gzip)
+			-- or standalone converters (bmp2png, bmp2gif)
+			if pack then os.execute(pack..fout) end
+		end -- if output f
 		inputf:close()
-		outputf:close()
-		-- here one may use either standard archivers (bzip2, gzip)
-		-- or standalone converters (bmp2png, bmp2gif)
-		if pack then os.execute(pack..fout) end
-	end
+	end -- if inputf
 end
 
 --[[ This function saves the fb-content (both 4bpp and 8bpp) as 8bpp PGM and pack it.