chk fp

because `\fp_use:N \undefined` eventually errors hard, this check needs to abort if it fails

l3kernel/l3fp-convert.dtx

@@ -544,7 +544,16 @@
 % \begin{macro}[EXP]{\fp_use:N, \fp_use:c, \fp_eval:n}
 %   Those public functions are simple copies of the decimal conversions.
 %    \begin{macrocode}
-\cs_new_eq:NN \fp_use:N \fp_to_decimal:N
+\__kernel_if_debug:TF
+  {
+    \cs_new:Npn \fp_use:N #1
+      {
+        \__kernel_chk_var_exist:NT #1 { \fp_to_decimal:N #1 }
+      }
+  }
+  {
+    \cs_new_eq:NN \fp_use:N \fp_to_decimal:N
+  }
 \cs_generate_variant:Nn \fp_use:N { c }
 \cs_new_eq:NN \fp_eval:n \fp_to_decimal:n
 %    \end{macrocode}