patch 8.2.3170: Illegal memory access in test

Problem:    Illegal memory access in test.
Solution:   Check pointer is not before the start of the line.
diff --git a/src/userfunc.c b/src/userfunc.c
index e21f15c..d8c943b 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -870,7 +870,7 @@
 	    end = p + STRLEN(p) - 1;
 	    while (end > p && VIM_ISWHITE(*end))
 		--end;
-	    if (*end == '{')
+	    if (end > p && *end == '{')
 	    {
 		--end;
 		while (end > p && VIM_ISWHITE(*end))