More defensive \scan_stop: (fixes #417)

Some past commit only added \scan_stop: to \dim_set:Nn, but there is also
\dim_add:Nn, \dim_set_eq:NN, \dim_zero:N and friends.

l3kernel/l3skip.dtx

@@ -1075,12 +1075,15 @@
 %
 % \begin{macro}{\dim_zero:N, \dim_zero:c}
 % \begin{macro}{\dim_gzero:N, \dim_gzero:c}
-%   Reset the register to zero.
+%   Reset the register to zero.  Using \cs{c_zero_skip} deals with the
+%   case where the variable passed is incorrectly a skip (for example a
+%   \LaTeXe{} length).
 %    \begin{macrocode}
 \__debug_patch:nnNNpn { \__debug_chk_var_local:N #1 } { }
-\cs_new_protected:Npn \dim_zero:N #1 { #1 \c_zero_dim }
+\cs_new_protected:Npn \dim_zero:N #1 { #1 \c_zero_skip }
 \__debug_patch:nnNNpn { \__debug_chk_var_global:N #1 } { }
-\cs_new_protected:Npn \dim_gzero:N #1 { \tex_global:D #1 \c_zero_dim }
+\cs_new_protected:Npn \dim_gzero:N #1
+  { \tex_global:D #1 \c_zero_skip }
 \cs_generate_variant:Nn \dim_zero:N  { c }
 \cs_generate_variant:Nn \dim_gzero:N { c }
 %    \end{macrocode}
@@ -1132,7 +1135,7 @@
 % \begin{macro}{\dim_gset:Nn, \dim_gset:cn}
 %   Setting dimensions is easy enough but when debugging we want both to
 %   check that the variable is correctly local/global and to wrap the
-%   expression in some code.  The |\scan_stop:| deals with the case
+%   expression in some code.  The \cs{scan_stop:} deals with the case
 %   where the variable passed is a skip (for example a \LaTeXe{}
 %   length).
 %    \begin{macrocode}
@@ -1151,14 +1154,17 @@
 % \begin{macro}{\dim_set_eq:NN, \dim_set_eq:cN, \dim_set_eq:Nc, \dim_set_eq:cc}
 % \begin{macro}
 %   {\dim_gset_eq:NN, \dim_gset_eq:cN, \dim_gset_eq:Nc, \dim_gset_eq:cc}
-%   All straightforward.
+%   All straightforward, with a \cs{scan_stop:} to deal with the case
+%   where |#1| is (incorrectly) a skip.
 %    \begin{macrocode}
 \__debug_patch:nnNNpn { \__debug_chk_var_local:N #1 } { }
-\cs_new_protected:Npn \dim_set_eq:NN #1#2 { #1 = #2 }
+\cs_new_protected:Npn \dim_set_eq:NN #1#2
+  { #1 = #2 \scan_stop: }
 \cs_generate_variant:Nn \dim_set_eq:NN {       c }
 \cs_generate_variant:Nn \dim_set_eq:NN { Nc , cc }
 \__debug_patch:nnNNpn { \__debug_chk_var_global:N #1 } { }
-\cs_new_protected:Npn \dim_gset_eq:NN #1#2 { \tex_global:D #1 = #2 }
+\cs_new_protected:Npn \dim_gset_eq:NN #1#2
+  { \tex_global:D #1 = #2 \scan_stop: }
 \cs_generate_variant:Nn \dim_gset_eq:NN {       c }
 \cs_generate_variant:Nn \dim_gset_eq:NN { Nc , cc }
 %    \end{macrocode}
@@ -1169,26 +1175,27 @@
 % \begin{macro}{\dim_gadd:Nn, \dim_gadd:cn}
 % \begin{macro}{\dim_sub:Nn, \dim_sub:cn}
 % \begin{macro}{\dim_gsub:Nn, \dim_gsub:cn}
-%   Using |by| here deals with the (incorrect) case |\dimen123|.  Since
+%   Using |by| here deals with the (incorrect) case |\dimen123|.
+%   Using \cs{scan_stop:} deals with skip variables.  Since
 %   debugging checks that the variable is correctly local/global, the
 %   global versions cannot be defined as \cs{tex_global:D} followed by
 %   the local versions.  The debugging code is inserted by
 %   \cs{@@_tmp:w}.
 %    \begin{macrocode}
 \@@_tmp:w \__debug_chk_var_local:N
 \cs_new_protected:Npn \dim_add:Nn #1#2
-  { \tex_advance:D #1 by \@@_eval:w #2 \@@_eval_end: }
+  { \tex_advance:D #1 by \@@_eval:w #2 \@@_eval_end: \scan_stop: }
 \@@_tmp:w \__debug_chk_var_global:N
 \cs_new_protected:Npn \dim_gadd:Nn #1#2
-  { \tex_global:D \tex_advance:D #1 by \@@_eval:w #2 \@@_eval_end: }
+  { \tex_global:D \tex_advance:D #1 by \@@_eval:w #2 \@@_eval_end: \scan_stop: }
 \cs_generate_variant:Nn \dim_add:Nn  { c }
 \cs_generate_variant:Nn \dim_gadd:Nn { c }
 \@@_tmp:w \__debug_chk_var_local:N
 \cs_new_protected:Npn \dim_sub:Nn #1#2
-  { \tex_advance:D #1 by - \@@_eval:w #2 \@@_eval_end: }
+  { \tex_advance:D #1 by - \@@_eval:w #2 \@@_eval_end: \scan_stop: }
 \@@_tmp:w \__debug_chk_var_global:N
 \cs_new_protected:Npn \dim_gsub:Nn #1#2
-  { \tex_global:D \tex_advance:D #1 by - \@@_eval:w #2 \@@_eval_end: }
+  { \tex_global:D \tex_advance:D #1 by - \@@_eval:w #2 \@@_eval_end: \scan_stop: }
 \cs_generate_variant:Nn \dim_sub:Nn  { c }
 \cs_generate_variant:Nn \dim_gsub:Nn { c }
 %    \end{macrocode}