fix size_t comparison of negative in file_read()

size_t is unsigned, so comparison against -1 effectively causes undefined behavior

src/utils/gravity_utils.c

@@ -147,7 +147,7 @@ const char *file_read(const char *path, size_t *len) {
     buffer[fsize] = 0;
     
     fsize2 = read(fd, buffer, (size_t)fsize);
-    if (fsize2 == -1) goto abort_read;
+    if (fsize2 != fsize) goto abort_read;
     
     if (len) *len = fsize2;
     close(fd);