patch 8.1.0919: compiler warnings

Problem:    Compiler warnings.
Solution:   Add type casts. (Mike Williams)
diff --git a/src/regexp_nfa.c b/src/regexp_nfa.c
index dadb9d0..b83e309 100644
--- a/src/regexp_nfa.c
+++ b/src/regexp_nfa.c
@@ -4800,7 +4800,7 @@
 		emsg(_(e_maxmempat));
 		return NULL;
 	    }
-	    newl = (nfa_thread_T *)alloc(newsize);
+	    newl = (nfa_thread_T *)alloc((int)newsize);
 	    if (newl == NULL)
 		return NULL;
 	    l->len = newlen;