Fix warning for unchecked errno in api.c test

Replace rewind() with fseek(f, 0, SEEK_SET) in the test helper
function. rewind() internally clears errno, which scan-build flags
as overwriting a previously unchecked errno value. fseek() to
offset 0 is functionally equivalent without the implicit errno
reset.

Author: ejohnstown

tests/api.c

@@ -522,7 +522,12 @@ static int load_file(const char* filename, byte** buf, word32* bufSz)
     }
 
     if (ret == 0) {
-        rewind(f);
+        ret = fseek(f, 0, XSEEK_SET);
+        if (ret < 0)
+            ret = -8;
+    }
+
+    if (ret == 0) {
         *buf = (byte*)malloc(*bufSz);
         if (*buf == NULL)
             ret = -5;