INT 21h AH=4Bh (EXEC) on return, put a nonzero value in AX (and AL) for a buggy version of Magic Pockets so the INTRO proceeds to the GAME properly

CHANGELOG

@@ -1,4 +1,9 @@
 NEXT VERSION
+  - INT 21h AH=4Bh EXEC: Real MS-DOS returns a nonzero value like 0x3E01.
+    A buggy version of Magic Pockets fails to call INT 21 AH=4Dh to read
+    the INTRO.EXE exit code and expects AL to just be nonzero to continue
+    to the game. So to make it work properly (kind of), return a nonzero
+    value. (joncampbell123).
   - Tandy machine type: Many registers are WRITE ONLY according to SX
     documentation, yet "Troubadours" likes to read/modify/write those
     registers in what appears to be hand-rolled manual modesetting code

src/dos/dos.cpp

@@ -2352,6 +2352,13 @@ static Bitu DOS_21Handler(void) {
             dos_program_running = false;
             *appname=0;
             *appargs=0;
+            /* Magic Pockets (slightly buggy version on the Internet Archive) expects AL
+             * to be nonzero to continue from INTRO.EXE to the game. The buggy version
+             * expects AL to be the INTRO.EXE error code but apparently they forgot to
+             * call INT 21h AH=4Dh to read it. Real MS-DOS appears to return something
+             * like AX=0x3E01 on return from INT 21h AH=4Bh. There is a non-buggy
+             * version on various abandonware sites that do not have this bug. */
+            reg_ax=0x3E01;
             break;
         case 0x4d:                  /* Get Return code */
             reg_al=dos.return_code;/* Officially read from SDA and clear when read */