libkiconv: address memory leak in not-found cases

Found in "Understanding and Detecting Disordered Error Handling with
Precise Function Pairing" by Qiushi Wu et al.

Reviewed by:	imp, jhb
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D29896

lib/libkiconv/kiconv_sysctl.c

@@ -59,6 +59,7 @@ kiconv_lookupconv(const char *drvname)
 				free(drivers);
 				return (0);
 			}
+		free(drivers);
 	}
 	return (ENOENT);
 }
@@ -86,6 +87,7 @@ kiconv_lookupcs(const char *tocode, const char *fromcode)
 				return (0);
 			}
 		}
+		free(csi);
 	}
 	return (ENOENT);
 }