Using return value of fwrite in posix_logger.h

Summary: Was causing error(warning) in third-party saying unused result

Test Plan: make

Reviewers: sheki, dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D9447

util/posix_logger.h

@@ -111,11 +111,12 @@ class PosixLogger : public Logger {
       }
 #endif
 
-      fwrite(base, 1, write_size, file_);
-      fflush(file_);
-
-      log_size_ += write_size;
-
+      size_t sz = fwrite(base, 1, write_size, file_);
+      assert(sz == write_size);
+      if (sz > 0) {
+        fflush(file_);
+        log_size_ += write_size;
+      }
       if (base != buffer) {
         delete[] base;
       }