Fix warning for unchecked errno in WREWIND macro

Replace rewind() with fseek(s, 0, SEEK_SET) in the WREWIND macro.
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.

wolfssh/port.h

@@ -449,7 +449,8 @@ extern "C" {
     #define WFSEEK(fs,s,o,w)    fseek((s),(o),(w))
     #define WFTELL(fs,s)        ftell((s))
     #define WFSTAT(fs,fd,b)     fstat((fd),(b))
-    #define WREWIND(fs,s)       rewind((s))
+    #define WREWIND(fs,s)       do { fseek((s),0,SEEK_SET); \
+                                     clearerr((s)); } while (0)
     #define WSEEK_END           SEEK_END
     #define WBADFILE            NULL
     #define WSETTIME(fs,f,a,m) (0)