patch 8.2.5077: various warnings from clang on MS-Windows

Problem:    Various warnings from clang on MS-Windows.
Solution:   Avoid the warnings. (Yegappan Lakshmanan, closes #10553)
diff --git a/src/misc1.c b/src/misc1.c
index 9bbe3ad..fd85eff 100644
--- a/src/misc1.c
+++ b/src/misc1.c
@@ -1351,6 +1351,9 @@
     int		mustfree;	// var was allocated, need to free it later
     int		at_start = TRUE; // at start of a name
     int		startstr_len = 0;
+#if defined(BACKSLASH_IN_FILENAME) || defined(AMIGA)
+    char_u	*save_dst = dst;
+#endif
 
     if (startstr != NULL)
 	startstr_len = (int)STRLEN(startstr);
@@ -1575,7 +1578,7 @@
 		// with it, skip a character
 		if (after_pathsep(dst, dst + c)
 #if defined(BACKSLASH_IN_FILENAME) || defined(AMIGA)
-			&& dst[-1] != ':'
+			&& (dst == save_dst || dst[-1] != ':')
 #endif
 			&& vim_ispathsep(*tail))
 		    ++tail;