msvcrt: Make _flsbuf thread safe.

dlls/msvcrt/file.c

@@ -3025,6 +3025,8 @@ int CDECL MSVCRT_fputc(int c, MSVCRT_FILE* file)
  */
 int CDECL MSVCRT__flsbuf(int c, MSVCRT_FILE* file)
 {
+    MSVCRT__lock_file(file);
+
     /* Flush output buffer */
     if(file->_bufsiz == 0 && !(file->_flag & MSVCRT__IONBF)) {
         msvcrt_alloc_buffer(file);
@@ -3033,20 +3035,28 @@ int CDECL MSVCRT__flsbuf(int c, MSVCRT_FILE* file)
         if(file->_flag & MSVCRT__IORW) {
             file->_flag |= MSVCRT__IOWRT;
         } else {
+            MSVCRT__unlock_file(file);
             return MSVCRT_EOF;
         }
     }
     if(file->_bufsiz) {
         int res=msvcrt_flush_buffer(file);
-	return res?res : MSVCRT_fputc(c, file);
+        if(!res)
+            res = MSVCRT_fputc(c, file);
+        MSVCRT__unlock_file(file);
+        return res;
     } else {
         unsigned char cc=c;
         int len;
         /* set _cnt to 0 for unbuffered FILEs */
         file->_cnt = 0;
         len = MSVCRT__write(file->_file, &cc, 1);
-        if (len == 1) return c & 0xff;
+        if (len == 1) {
+            MSVCRT__unlock_file(file);
+            return c & 0xff;
+        }
         file->_flag |= MSVCRT__IOERR;
+        MSVCRT__unlock_file(file);
         return MSVCRT_EOF;
     }
 }