Two sub bugs fixed; two new gsub tests have been added.

./jq --run-tests < tests/onig.test
has been run successfully:

...
Testing 'gsub("a";"b")' at line number 70
Testing 'gsub( "(.*)"; "";  "x")' at line number 74
...
22 of 22 tests passed (0 malformed)

tests/onig.test

@@ -67,6 +67,14 @@ gsub("(?<d>\\d)"; ":\(.d);")
 "a1b2"
 "a:1;b:2;"
 
+gsub("a";"b")
+"aaaaa"
+"bbbbb"
+
+gsub( "(.*)"; "";  "x")
+""
+""
+
 [.[] | scan(", ")]
 ["a,b, c, d, e,f",", a,b, c, d, e,f, "]
 [", ",", ",", ",", ",", ",", ",", ",", "]