Lib: check return value of fcntl() in boinc_fopen()

There is no way to communicate what failed to the caller because they usually don't check the return value in the case of non-zero.
fixes CID 27690 found by Coverity

lib/filesys.cpp

@@ -504,7 +504,9 @@ FILE* boinc_fopen(const char* path, const char* mode) {
         }
     }
     if (f) {
-        fcntl(fileno(f), F_SETFD, FD_CLOEXEC);
+        if (-1 == fcntl(fileno(f), F_SETFD, FD_CLOEXEC)) {
+            return 0;
+        }
     }
 #endif
     return f;