patch 8.0.0665: warning for uninitialized variable

Problem:    Warning for uninitialized variable. (Tony Mechelynck)
Solution:   Initialize it.
diff --git a/src/term.c b/src/term.c
index 555300b..f36f093 100644
--- a/src/term.c
+++ b/src/term.c
@@ -3944,7 +3944,7 @@
     int		offset;
     char_u	key_name[2];
     int		modifiers;
-    char_u	*modifiers_start;
+    char_u	*modifiers_start = NULL;
     int		key;
     int		new_slen;
     int		extra;