patch 8.0.1481: clearing a pointer takes two lines

Problem:    Clearing a pointer takes two lines.
Solution:   Add vim_clear() to free and clear the pointer.
diff --git a/src/proto/misc2.pro b/src/proto/misc2.pro
index 6999683..4c20a7c 100644
--- a/src/proto/misc2.pro
+++ b/src/proto/misc2.pro
@@ -46,6 +46,7 @@
 void vim_strcat(char_u *to, char_u *from, size_t tosize);
 int copy_option_part(char_u **option, char_u *buf, int maxlen, char *sep_chars);
 void vim_free(void *x);
+void vim_clear(void **x);
 int vim_stricmp(char *s1, char *s2);
 int vim_strnicmp(char *s1, char *s2, size_t len);
 char_u *vim_strchr(char_u *string, int c);