fix handling of empty environment

Currently if the DOS environment is empty (can happen
when starting command.com), djgpp startup produces the
corrupted environment. This patch fixes it by checking
the loop end condition before loop action.

Another problematic case is the missing environment segment.
In this case there is no work-around, so the error msg
is printed and the program terminates.

src/libc/crt0/crt1.c

@@ -128,31 +128,33 @@ char *__dos_argv0;
 static void
 setup_environment(void)
 {
-  char *dos_environ = alloca(_stubinfo->env_size), *cp;
+  char *dos_environ, *cp;
   short env_selector;
   int env_count=0;
+
+  dos_environ = alloca(_stubinfo->env_size);
   movedata(_stubinfo->psp_selector, 0x2c, ds, (int)&env_selector, 2);
   movedata(env_selector, 0, ds, (int)dos_environ, _stubinfo->env_size);
   cp = dos_environ;
-  do {
+  while (*cp) { /* repeat until two NULs */
     env_count++;
     while (*cp) cp++; /* skip to NUL */
     cp++; /* skip to next character */
-  } while (*cp); /* repeat until two NULs */
+  }
   _environ = (char **)malloc((env_count+1) * sizeof(char *));
   if (_environ == 0)
     return;
 
   cp = dos_environ;
   env_count = 0;
-  do {
+  while (*cp) { /* repeat until two NULs */
     /* putenv assumes each string is malloc'd */
     _environ[env_count] = (char *)malloc(strlen(cp)+1);
     strcpy(_environ[env_count], cp);
     env_count++;
     while (*cp) cp++; /* skip to NUL */
     cp++; /* skip to next character */
-  } while (*cp); /* repeat until two NULs */
+  }
   _environ[env_count] = 0;
 
   /*

src/stub/stub.asm

@@ -160,13 +160,21 @@ resize_again:
 ;  Scan environment for "PATH=" and the stub's full name after environment
 
 	mov	es, es:[0x2c]		; get environment segment
+	mov	di, es
+	or	di, di			; check if no env
+	jnz	@f1
+	mov	al, 111
+	mov	dx, msg_no_env
+	jmpl	error
+@f1:
 	xor	di, di			; begin search for NUL/NUL (di = 0)
 ;	mov	cx, 0xff04		; effectively `infinite' loop
 	xor 	al, al
-	.db	0xa9			; "test ax,...." -- skip 2 bytes
+	jmp	@f1
 scan_environment:
 	repne
 	scasb				; search for NUL
+@f1:
 	cmpw	es:[di], 0x4150		; "PA"
 	jne	not_path
 	scasw
@@ -182,6 +190,8 @@ not_path:
 	scasb
 	jne	scan_environment	; no, still environment
 	scasw				; adjust pointer to point to prog name
+	push	es
+	push	di
 
 ;; When we are spawned from a program which has more than 20 handles in use,
 ;; all the handles passed to us by DOS are taken (since only the first 20
@@ -199,8 +209,6 @@ not_path:
 ;-----------------------------------------------------------------------------
 ;  Get DPMI information before doing anything 386-specific
 
-	push	es
-	push	di
 	xor	cx, cx			; flag for load attempt set cx = 0
 	jz	@f2			; We always jump, shorter than jmp
 @b1:
@@ -828,6 +836,8 @@ msg_not_exe:
 	.db	": not EXE$"
 msg_not_coff:
 	.db	": not COFF (Check for viruses)$"
+msg_no_env:
+	.db	"no envseg$"
 msg_no_dpmi:
 	.db	"no DPMI - Get csdpmi*b.zip$"
 msg_no_dos_memory: