change regexp_interal attribute from I32 to U32

This changes the name_list_idx attribute from I32 to a U32 as
it will never be negative, and as of a963d6d a 0 can be
safely used to represent "no value" for items in the 'data' array.

Author: demerphq

pod/perlreguts.pod

@@ -832,7 +832,7 @@ value to other engine implementations.
         struct reg_data *data;
         struct reg_code_blocks *code_blocks;
         U32 proglen;
-        int name_list_idx;
+        U32 name_list_idx;
         regnode program[1];
     } regexp_internal;
 

regcomp.h

@@ -71,7 +71,7 @@ typedef struct regexp_internal {
                                    strictly reserved for internal purposes. */
         struct reg_code_blocks *code_blocks;/* positions of literal (?{}) */
         U32 proglen;            /* size of the compiled program in regnodes */
-        int name_list_idx;        /* Optional data index of an array of paren names */
+        U32 name_list_idx;        /* Optional data index of an array of paren names */
         regnode program[1];	/* Unwarranted chumminess with compiler. */
 } regexp_internal;