shlwapi: Avoid buffer overflow in PathCombineA.

dlls/shlwapi/path.c

@@ -155,20 +155,21 @@ LPSTR WINAPI PathCombineA(LPSTR lpszDest, LPCSTR lpszDir, LPCSTR lpszFile)
   if (!lpszDest)
     return NULL;
   if (!lpszDir && !lpszFile)
-  {
-    lpszDest[0] = 0;
-    return NULL;
-  }
+    goto fail;
 
   if (lpszDir)
-    MultiByteToWideChar(CP_ACP,0,lpszDir,-1,szDir,MAX_PATH);
+    if (!MultiByteToWideChar(CP_ACP,0,lpszDir,-1,szDir,MAX_PATH))
+      goto fail;
+
   if (lpszFile)
-    MultiByteToWideChar(CP_ACP,0,lpszFile,-1,szFile,MAX_PATH);
+    if (!MultiByteToWideChar(CP_ACP,0,lpszFile,-1,szFile,MAX_PATH))
+      goto fail;
 
   if (PathCombineW(szDest, lpszDir ? szDir : NULL, lpszFile ? szFile : NULL))
     if (WideCharToMultiByte(CP_ACP,0,szDest,-1,lpszDest,MAX_PATH,0,0))
       return lpszDest;
 
+fail:
   lpszDest[0] = 0;
   return NULL;
 }