Don’t duplicate error checks already done in source functions.

lib/zip_fseek.c

diff --git a/lib/zip_fseek.c b/lib/zip_fseek.c
index 83d40dd..a16af1a 100644
--- a/lib/zip_fseek.c
+++ b/lib/zip_fseek.c
@@ -45,16 +45,6 @@
     if (zf->error.zip_err != 0)
 	return -1;
 
-    if (whence != SEEK_SET && whence != SEEK_CUR && whence != SEEK_END) {
-	zip_error_set(&zf->error, ZIP_ER_INVAL, 0);
-	return -1;
-    }
-
-    if ((zip_source_supports(zf->src) & ZIP_SOURCE_MAKE_COMMAND_BITMASK(ZIP_SOURCE_SEEK)) == 0) {
-	zip_error_set(&zf->error, ZIP_ER_OPNOTSUPP, 0);
-	return -1;
-    }
-
     if (zip_source_seek(zf->src, offset, whence) < 0) {
 	_zip_error_set_from_source(&zf->error, zf->src);
 	return -1;

lib/zip_ftell.c

diff --git a/lib/zip_ftell.c b/lib/zip_ftell.c
index 895ce81..abbf296 100644
--- a/lib/zip_ftell.c
+++ b/lib/zip_ftell.c
@@ -46,11 +46,6 @@
     if (zf->error.zip_err != 0)
 	return -1;
 
-    if ((zip_source_supports(zf->src) & ZIP_SOURCE_MAKE_COMMAND_BITMASK(ZIP_SOURCE_TELL)) == 0) {
-	zip_error_set(&zf->error, ZIP_ER_OPNOTSUPP, 0);
-	return -1;
-    }
-
     res = zip_source_tell(zf->src);
     if (res < 0) {
 	_zip_error_set_from_source(&zf->error, zf->src);