LibGfx/JBIG2: Fix size bound in scan_for_immediate_generic_region_size()

The memmem() call passes `data.size() - 19 - sizeof(u32)` for big_len,
(18 prefix bytes skipped, the flag byte, and the trailing u32), so the
buffer needs to be at least that large.

Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=67332

Userland/Libraries/LibGfx/ImageFormats/JBIG2Loader.cpp

@@ -306,8 +306,8 @@ static ErrorOr<size_t> scan_for_immediate_generic_region_size(ReadonlyBytes data
     //  Thus, those sequences cannot occur by chance in the data that is decoded to generate the contents of the generic region."
     dbgln_if(JBIG2_DEBUG, "(Unknown data length, computing it)");
 
-    if (data.size() < 18)
-        return Error::from_string_literal("JBIG2ImageDecoderPlugin: Data too short to contain segment data header");
+    if (data.size() < 19 + sizeof(u32))
+        return Error::from_string_literal("JBIG2ImageDecoderPlugin: Data too short to contain segment data header and end sequence");
 
     // Per 7.4.6.1 Generic region segment data header, this starts with the 17 bytes described in
     // 7.4.1 Region segment information field, followed the byte described in 7.4.6.2 Generic region segment flags.