patch 8.1.0641: no check for out-of-memory when converting regexp

Problem:    No check for out-of-memory when converting regexp.
Solution:   Bail out when lalloc() returns NULL. (John Marriott)

src/regexp_nfa.c

@@ -3218,8 +3218,10 @@ post2nfa(int *postfix, int *end, int nfa_calc_size)
 
     if (nfa_calc_size == FALSE)
     {
-	/* Allocate space for the stack. Max states on the stack : nstate */
+	// Allocate space for the stack. Max states on the stack: "nstate'.
 	stack = (Frag_T *)lalloc((nstate + 1) * sizeof(Frag_T), TRUE);
+	if (stack == NULL)
+	    return NULL;
 	stackp = stack;
 	stack_end = stack + (nstate + 1);
     }

src/version.c

@@ -799,6 +799,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    641,
 /**/
     640,
 /**/