[Coco] mess video bug

[Earl Casper]

Th following program demonstrates a flaw in mess when placing the hi-res
text buffer anywhere but $36

10 CLEAR 200,&H5000:WIDTH 40:LOADM"TEST1:0":EXEC

00100  ORG $5000
00110 START LDA #$BC
00120  STA $FF9D
00130  LDA #$2F
00140  STA $FFA3
00150  LDX #MESG
00160  LDU #$6000+20
00170 LOOP LDA ,X+
00180  STA ,U++
00190  BNE LOOP
00200 WAIT JSR [$A000]
00210  BEQ WAIT
00220  LDD #$D803
00230  STD $FF9D
00240  LDA #$36
00250  STA $FFA3
00260  LDX #MESG
00270  LDU #$6000+20+24
00280 LOOP1 LDA ,X+
00290  STA ,U++
00300  BNE LOOP1
00310 WAIT1 JSR [$A000]
00320  BEQ WAIT1
00330  RTS
00340 MESG FCC /THIS IS A TEST./
00350  FCB 0
00360  END START