patch 9.0.0552: crash when using NUL in buffer that uses :source

Problem:    Crash when using NUL in buffer that uses :source.
Solution:   Don't get a next line when skipping over NL.
diff --git a/src/eval.c b/src/eval.c
index 1f43970..77f0d7f 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -253,7 +253,7 @@
 	    if (fc == NULL)
 		return FAIL;
 
-	    // Shortcut to call a compiled function without overhead.
+	    // Shortcut to call a compiled function with minimal overhead.
 	    r = call_def_function(partial->pt_func, argc, argv,
 					  DEF_USE_PT_ARGV, partial, fc, rettv);
 	    remove_funccal();
@@ -2291,7 +2291,7 @@
 
 	if (next != NULL)
 	{
-	    *getnext = TRUE;
+	    *getnext = *p != NL;
 	    return skipwhite(next);
 	}
     }