patch 8.2.4289: warnings reported by MSVC

Problem:    Warnings reported by MSVC.
Solution:   Rename variables and other fixes. (Ken Takata, closes #9689)
diff --git a/src/filepath.c b/src/filepath.c
index 65ea2bc..4abf8f4 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -372,12 +372,12 @@
 	    {
 		if (GetLongPathNameW(wfname, buf, _MAX_PATH))
 		{
-		    char_u *p = utf16_to_enc(buf, NULL);
+		    char_u *q = utf16_to_enc(buf, NULL);
 
-		    if (p != NULL)
+		    if (q != NULL)
 		    {
 			vim_free(*bufp);    // free any allocated file name
-			*bufp = *fnamep = p;
+			*bufp = *fnamep = q;
 		    }
 		}
 		vim_free(wfname);