patch 8.2.4637: warning for using uninitialized variable

Problem:    Warning for using uninitialized variable. (Tony Mechelynck)
Solution:   Initialize it.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index ab2aa60..c5e96bd 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -2782,7 +2782,7 @@
 	cmdmod_T    *cmod,
 	int	    skip_only)
 {
-    char_u  *cmd_start;
+    char_u  *cmd_start = NULL;
     char_u  *p;
     int	    starts_with_colon = FALSE;
     int	    vim9script = in_vim9script();