asm/preproc.c: fix NULL pointer on %exitrep outside %rep

When %exitrep incorrectly occurs outside a %rep block, do_exit_macro()
returns NULL, but the %exitrep code would try to set m->in_progress =
1 anyway, causing a NULL pointer dereference and crashing NASM.

Add a NULL pointer guard around this assignment; an error has already
been issued.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

asm/preproc.c

diff --git a/asm/preproc.c b/asm/preproc.c
index 8e876a1..75e4aac 100644
--- a/asm/preproc.c
+++ b/asm/preproc.c
@@ -5204,7 +5204,8 @@
     case PP_EXITREP:
     {
         MMacro *m = do_exit_macro(dname, false);
-        m->in_progress = 1;     /* No more repeats */
+        if (m)
+            m->in_progress = 1;     /* No more repeats */
         break;
     }