patch 8.0.1397: pattern with \& following nothing gives an error

Problem:    Pattern with \& following nothing gives an error.
Solution:   Emit an empty node when needed.

src/regexp_nfa.c

@@ -2321,7 +2321,6 @@ nfa_regconcat(void)
     static int
 nfa_regbranch(void)
 {
-    int		ch;
     int		old_post_pos;
 
     old_post_pos = (int)(post_ptr - post_start);
@@ -2330,11 +2329,13 @@ nfa_regbranch(void)
     if (nfa_regconcat() == FAIL)
 	return FAIL;
 
-    ch = peekchr();
     /* Try next concats */
-    while (ch == Magic('&'))
+    while (peekchr() == Magic('&'))
     {
 	skipchr();
+	/* if concat is empty do emit a node */
+	if (old_post_pos == (int)(post_ptr - post_start))
+	    EMIT(NFA_EMPTY);
 	EMIT(NFA_NOPEN);
 	EMIT(NFA_PREV_ATOM_NO_WIDTH);
 	old_post_pos = (int)(post_ptr - post_start);
@@ -2344,7 +2345,6 @@ nfa_regbranch(void)
 	if (old_post_pos == (int)(post_ptr - post_start))
 	    EMIT(NFA_EMPTY);
 	EMIT(NFA_CONCAT);
-	ch = peekchr();
     }
 
     /* if a branch is empty, emit one node for it */

src/testdir/test_search.vim

@@ -721,3 +721,11 @@ func Test_search_multibyte()
   enew!
   let &encoding = save_enc
 endfunc
+
+" This was causing E874.  Also causes an invalid read?
+func Test_look_behind()
+  new
+  call setline(1, '0\|\&\n\@<=') 
+  call search(getline("."))
+  bwipe!
+endfunc

src/version.c

@@ -771,6 +771,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1397,
 /**/
     1396,
 /**/