Fixed sign-compare warning

In the ParseChain function the bufLen and pos varaible were
changed from int to size_t.  Every location that ParseChain
is called is passing in a size_t as the bufLen an unsigned
type. The pos variable is never negative and should be the
same type as the bufLen.

While in the function the documentation was cleaned up and
comemnts were added to quickly identify the contents of the
pemCertHeader and pemCerFooter byte arrays. As well as clarify
the need of the goto `exit:` label when using the
CHECK_MBEDTLS_RET macro.

Change-Id: I1f063dddea29a2a00ae6e47c7b2240794bcaf5bb
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/13519Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Dmitriy Zhuravlev <d.zhuravlev@samsung.com>
Reviewed-by: Rick Bell <richard.s.bell@intel.com>

resource/csdk/connectivity/src/adapter_util/ca_adapter_net_ssl.c

@@ -186,6 +186,9 @@ if (0 != (ret) && MBEDTLS_ERR_SSL_WANT_READ != (int) (ret) &&
  *
  * If function returns error code it goes to error processing.
  *
+ * **IMPORTANT:** Any time CHECK_MBEDTLS_RET is used an `exit:` goto label must
+ *                be present to handle error processing.
+ *
  * @param[in] f  Function to call
  */
 #define CHECK_MBEDTLS_RET(f, ...) do {                                                             \
@@ -529,27 +532,27 @@ static int RecvCallBack(void * tep, unsigned char * data, size_t dataLen)
  * Parse chain of X.509 certificates.
  *
  * @param[out] crt     container for X.509 certificates
- * @param[in]  data    buffer with X.509 certificates. Certificates may be in either in PEM
+ * @param[in]  buf     buffer with X.509 certificates. Certificates may be in either in PEM
                        or DER format in a jumble. Each PEM certificate must be NULL-terminated.
  * @param[in]  bufLen  buffer length
  *
  * @return  0 on success, -1 on error
  */
-static int ParseChain(mbedtls_x509_crt * crt, const unsigned char * buf, int bufLen)
+static int ParseChain(mbedtls_x509_crt * crt, const unsigned char * buf, size_t bufLen)
 {
     OIC_LOG_V(DEBUG, NET_SSL_TAG, "In %s", __func__);
     VERIFY_NON_NULL_RET(crt, NET_SSL_TAG, "Param crt is NULL" , -1);
     VERIFY_NON_NULL_RET(buf, NET_SSL_TAG, "Param buf is NULL" , -1);
 
-    int pos = 0;
-    int ret = 0;
+    size_t pos = 0;
     size_t len = 0;
     unsigned char * tmp = NULL;
-
+    /* byte encoded ASCII string '-----BEGIN CERTIFICATE-----' */
     char pemCertHeader[] = {
         0x2d, 0x2d, 0x2d, 0x2d, 0x2d, 0x42, 0x45, 0x47, 0x49, 0x4e, 0x20, 0x43, 0x45, 0x52,
         0x54, 0x49, 0x46, 0x49, 0x43, 0x41, 0x54, 0x45, 0x2d, 0x2d, 0x2d, 0x2d, 0x2d
     };
+    // byte encoded ASCII string '-----END CERTIFICATE-----' */
     char pemCertFooter[] = {
         0x2d, 0x2d, 0x2d, 0x2d, 0x2d, 0x45, 0x4e, 0x44, 0x20, 0x43, 0x45, 0x52, 0x54, 0x49,
         0x46, 0x49, 0x43, 0x41, 0x54, 0x45, 0x2d, 0x2d, 0x2d, 0x2d, 0x2d
@@ -602,14 +605,13 @@ static int ParseChain(mbedtls_x509_crt * crt, const unsigned char * buf, int buf
         else
         {
              OIC_LOG_BUFFER(DEBUG, NET_SSL_TAG, buf, bufLen);
-             OIC_LOG_V(ERROR, NET_SSL_TAG, "parseChain returned -0x%x", -ret);
              OIC_LOG_V(DEBUG, NET_SSL_TAG, "Out %s", __func__);
              return -1;
         }
     }
     OIC_LOG_V(DEBUG, NET_SSL_TAG, "Out %s", __func__);
     return 0;
-
+    // exit label required for CHECK_MBEDTLS_RET macro
 exit:
     return -1;
 }
@@ -1978,7 +1980,7 @@ static int pHash (const unsigned char *key, size_t keyLen,
     mbedtls_md_free(&hmacA);
     mbedtls_md_free(&hmacP);
     return bufLen;
-
+    // exit label required for CHECK_MBEDTLS_RET macro
 exit:
     mbedtls_md_free(&hmacA);
     mbedtls_md_free(&hmacP);