pal16bxcl is an array of 256 dwords, not bytes:
    src/endmem.asm:NEWSYM pal16bxcl, resd 256

While at it fixes off-by-4 out of bounds exit.

Detected by _FORTIFY_SOURCE=3:
    *** buffer overflow detected ***: terminated
    #7  0x08057c14 in memset (__len=2, __ch=255, __dest=<optimized out>) at ...-glibc-2.38-23-dev/include/bits/string_fortified.h:59
#8  clearmem () at initc.c:1461
--- a/src/initc.c
+++ b/src/initc.c
@@ -1389,7 +1389,7 @@ extern unsigned char vidmemch8[4096];
 extern unsigned char pal16b[1024];
 extern unsigned char pal16bcl[1024];
 extern unsigned char pal16bclha[1024];
-extern unsigned char pal16bxcl[256];
+extern unsigned char pal16bxcl[1024];
 extern unsigned char SPCRAM[65472];
 unsigned char *SPCState = SPCRAM;