LibGfx: Second attempt to handle max_symbol correctly in webp decoder

The previous attempt was in commit e5e9d3b877c, where I thought
max_symbol describes how many code lengths should be read.

But it looks like it instead describes how many code length input
symbols should be read. (The two aren't the same since one code length
input symbol can produce several code lengths.)

I still agree with the commit description of e5e9d3b877c that the spec
isn't very clear on this :)

This time I've found a file that sets max_symbol and with this change
here, that file decodes correctly. (It's Qpalette.webp, which I'm about
to add as a test case.)

Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp

@@ -438,8 +438,12 @@ static ErrorOr<CanonicalCode> decode_webp_chunk_VP8L_prefix_code(WebPLoadingCont
     u8 last_non_zero = 8; // "If code 16 is used before a non-zero value has been emitted, a value of 8 is repeated."
 
     // "A prefix table is then built from code_length_code_lengths and used to read up to max_symbol code lengths."
-    dbgln_if(WEBP_DEBUG, "  reading {} symbols", max_symbol);
-    while (code_lengths.size() < max_symbol) {
+    dbgln_if(WEBP_DEBUG, "  reading {} symbols from at most {} codes", alphabet_size, max_symbol);
+    while (code_lengths.size() < alphabet_size) {
+        if (max_symbol == 0)
+            break;
+        --max_symbol;
+
         auto symbol = TRY(code_length_code.read_symbol(bit_stream));
 
         if (symbol < 16) {
@@ -472,8 +476,8 @@ static ErrorOr<CanonicalCode> decode_webp_chunk_VP8L_prefix_code(WebPLoadingCont
         }
     }
 
-    if (code_lengths.size() != alphabet_size)
-        return Error::from_string_literal("Number of code lengths does not match the sum of codes");
+    if (code_lengths.size() > alphabet_size)
+        return Error::from_string_literal("Number of code lengths is larger than the alphabet size");
 
     dbgln_if(WEBP_DEBUG, "  done reading symbols");
     return CanonicalCode::from_bytes(code_lengths);