preproc: avoid crashing on %exitmacro in broken nesting

Broken %if, %rep and %macro nesting can result in the %exitmacro
unwind overrunning the condition stack. Fix.

Fixes: https://bugzilla.nasm.us/show_bug.cgi?id=3392796
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

Author: H. Peter Anvin

asm/preproc.c

@@ -4570,6 +4570,10 @@ static int do_directive(Token *tline, Token **output)
              */
             for ( ; l->finishes->condcnt > 0; l->finishes->condcnt --) {
                 cond = istk->conds;
+                if (!cond) {
+                    l->finishes->condcnt = 0;
+                    break;      /* Possible in case of invalid nesting */
+                }
                 istk->conds = cond->next;
                 nasm_free(cond);
             }