Fix iterration problem for non decimal string

When the string transformation to number failed, all following
transformation failed too.

This happend because status in decNumberFromString function is
updated just in error case. Reusing the DEC_CONTEXT that failed
before results into error even if the string is valid number.

tests/man.test

@@ -841,3 +841,8 @@ true
 {"foo": 42}
 {"foo": 43}
 
+.[]|tonumber?
+["1", "hello", "3", 4]
+1
+3
+4