Speed up cimg::fsize

Author: apocelipes

CImg.h

@@ -5799,11 +5799,11 @@ 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);
       struct _stati64 st;
-      if (fd>=0 && !_fstati64(fd,&st)) return (cimg_int64)st.st_size;
+      if (!_fstati64(fd,&st)) return (cimg_int64)st.st_size;
 #endif
 
       // Fallback, used for non-POSIX systems or if fstat failed (pipes or sockets).