Fix two silly bugs in stringprint.h
Fix two silly bugs in stringprint.h

Don't print double % signs when given "%%" formatting string.
Fixed logic related to Assert checking for "%%" in formatting strings.

shihchinw/pbrt-v3,ambakshi/pbrt-v3,shihchinw/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,ambakshi/pbrt-v3,nispaur/pbrt-v3,ambakshi/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,nispaur/pbrt-v3