cmListFileLexer: fix 'bail out on seek-errors' also in original file

The commit v3.9.0-rc1~502^2 (cmListFileLexer: bail out on seek-errors,
2017-02-19) for bug # 16607 was only applied to the generated file.

Also apply the fix now to the original.

Signed-off-by: Matthias Maennich <[email protected]>

Source/LexerParser/cmListFileLexer.in.l

@@ -439,11 +439,15 @@ static cmListFileLexer_BOM cmListFileLexer_ReadBOM(FILE* f)
       if (fread(b, 1, 2, f) == 2 && b[0] == 0 && b[1] == 0) {
         return cmListFileLexer_BOM_UTF32LE;
       }
-      fsetpos(f, &p);
+      if (fsetpos(f, &p) != 0) {
+        return cmListFileLexer_BOM_Broken;
+      }
       return cmListFileLexer_BOM_UTF16LE;
     }
   }
-  rewind(f);
+  if (fseek(f, 0, SEEK_SET) != 0) {
+    return cmListFileLexer_BOM_Broken;
+  }
   return cmListFileLexer_BOM_None;
 }