Speed up cimg::fsize

Author: apocelipes

CImg.h

@@ -67567,13 +67567,11 @@ namespace cimg_library {
 #if cimg_OS==1 // Optimized for POSIX Environements (Linux, macOS, BSD, etc.)
       const int fd = fileno(file);
       struct stat st;
-      if (fd>=0 && !fstat(fd,&st)) return (cimg_int64)st.st_size;
+      if (!fstat(fd,&st)) return (cimg_int64)st.st_size;
 #elif cimg_OS==2 // Optimized for Windows Environements (MSVC, MinGW)
       const int fd = _fileno(file);
-      if (fd>=0) {
-        const cimg_int64 siz = (cimg_int64)_filelengthi64(fd);
-        if (siz>=0) return siz;
-      }
+      const cimg_int64 siz = (cimg_int64)_filelengthi64(fd);
+      if (siz>=0) return siz;
 #endif
       // Fallback, used for non-POSIX systems or if fstat failed (pipes or sockets).
       const cimg_long pos = cimg::ftell(file);