Add 0th capture to RegExp.prototype bytecode

The bytecode was missing DUK_REOP_SAVE for indices 0 and 1, i.e. the
zeroth capture, which caused the RegExp.prototype to match an empty
input but without providing the zeroth capture which is assumed in
other parts of Duktape internals.  This caused the GH-492 assert failure
and, with asserts disabled, a segfault.

src/genbuiltins.py

@@ -753,7 +753,7 @@ bi_regexp_prototype = {
 		# RegExp internal value should match that of new RegExp() (E5 Sections 15.10.6
 		# and 15.10.7), i.e. a bytecode sequence that matches an empty string.
 		# The compiled regexp bytecode for that is embedded here, and must match the
-		# defines in duk_regexp.h.
+		# defines in duk_regexp.h.  The bytecode must provide the 0'th capture.
 		#
 		# Note that the property attributes are non-default.
 
@@ -762,6 +762,10 @@ bi_regexp_prototype = {
 			'name': internal('Bytecode'),
 			'value': unichr(0) +		# flags (none)
 			         unichr(2) +		# nsaved == 2
+			         unichr(11) +           # DUK_REOP_SAVE
+			         unichr(0) +            # 0
+			         unichr(11) +           # DUK_REOP_SAVE
+			         unichr(1) +            # 1
 			         unichr(1),		# DUK_REOP_MATCH
 			'attributes': '',
 		},