patch 8.0.1803: warning for uninitialized variable

Problem:    Warning for uninitialized variable. (Tony Mechelynck)
Solution:   Initialize it.
diff --git a/src/terminal.c b/src/terminal.c
index 4dd6665..0c483e1 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -4758,7 +4758,7 @@
 f_term_gettty(typval_T *argvars, typval_T *rettv)
 {
     buf_T	*buf = term_get_buf(argvars, "term_gettty()");
-    char_u	*p;
+    char_u	*p = NULL;
     int		num = 0;
 
     rettv->v_type = VAR_STRING;