ltrimstr/1+rtrimstr/1: don't leak on invalid input or arguments

ltrimstr/rtrimstr was ignoring and leaking the error returned by
f_startswith()/f_endswith().

This also means that they just let the input pass through for non-string
inputs or arguments.

Only fix the leak for now; in the next release, #2969 will make them
rethrow the error returned by startswith/endswith.

Ref: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=64946

tests/jq.test

@@ -2091,7 +2091,22 @@ try ("foobar" | .[1.5]) catch .
 null
 "Cannot index string with number"
 
+
 # setpath/2 does not leak the input after an invalid get #2970
+
 try ["ok", setpath([1]; 1)] catch ["ko", .]
 {"hi":"hello"}
 ["ko","Cannot index object with number"]
+
+
+# ltrimstr/1 rtrimstr/1 don't leak on invalid input #2977
+
+try ltrimstr(1) catch "x", try rtrimstr(1) catch "x" | "ok"
+"hi"
+"ok"
+"ok"
+
+try ltrimstr("x") catch "x", try rtrimstr("x") catch "x" | "ok"
+{"hey":[]}
+"ok"
+"ok"