pbx_variables: Use const char if possible.

Use const char for char arguments to
pbx_substitute_variables_helper_full_location
that can do so (context and exten).

ASTERISK-30209 #close

Change-Id: I001357177e9c3dca2b2b4eebc5650c1095b3da6f

include/asterisk/pbx.h

@@ -1432,7 +1432,7 @@ void pbx_substitute_variables_helper_full(struct ast_channel *c, struct varshead
 /*!
  * \brief Substitutes variables, similar to pbx_substitute_variables_helper_full, but allows passing the context, extension, and priority in.
  */
-void pbx_substitute_variables_helper_full_location(struct ast_channel *c, struct varshead *headp, const char *cp1, char *cp2, int cp2_size, size_t *used, char *context, char *exten, int pri);
+void pbx_substitute_variables_helper_full_location(struct ast_channel *c, struct varshead *headp, const char *cp1, char *cp2, int cp2_size, size_t *used, const char *context, const char *exten, int pri);
 /*! @} */
 
 /*! @name Substitution routines, using dynamic string buffers

main/pbx_variables.c

@@ -662,7 +662,7 @@ void pbx_substitute_variables_helper_full(struct ast_channel *c, struct varshead
 	pbx_substitute_variables_helper_full_location(c, headp, cp1, cp2, count, used, NULL, NULL, 0);
 }
 
-void pbx_substitute_variables_helper_full_location(struct ast_channel *c, struct varshead *headp, const char *cp1, char *cp2, int count, size_t *used, char *context, char *exten, int pri)
+void pbx_substitute_variables_helper_full_location(struct ast_channel *c, struct varshead *headp, const char *cp1, char *cp2, int count, size_t *used, const char *context, const char *exten, int pri)
 {
 	/* Substitutes variables into cp2, based on string cp1, cp2 NO LONGER NEEDS TO BE ZEROED OUT!!!!  */
 	const char *whereweare;