Memory leak fix for HMAC verification

This commit commit fixes a memory leak in the HMAC verification code found with
the test suite running in valgrind mode.  Here is the './test-fwknop.pl --diff'
output showing fko_verify_hmac() removed from the flagged functions list:

 [+] fwknop functions (unique view):
-        8 : ???
-        3 : main
-        3 : pcap_capture
-        1 : incoming_spa
+        7 : ???
+        2 : pcap_capture
+        2 : main
         1 : pcap_compile
-        1 : fko_new_with_data
-        1 : strndup
-        1 : fko_verify_hmac

 [+] fwknop functions (with call line numbers):
-        8 : ??? (in /usr/lib/x86_64-linux-gnu/libpcap.so.1.1.1)
-        3 : main (fwknopd.c:299)
-        1 : fko_new_with_data (fko_funcs.c:220)
-        1 : pcap_capture (pcap_capture.c:105)
-        1 : incoming_spa (incoming_spa.c:376)
-        1 : strndup (strndup.c:46)
+        7 : ??? (in /usr/lib/x86_64-linux-gnu/libpcap.so.1.1.1)
+        2 : main (fwknopd.c:299)
         1 : pcap_compile (in /usr/lib/x86_64-linux-gnu/libpcap.so.1.1.1)
-        1 : pcap_capture (pcap_capture.c:226)
         1 : pcap_capture (pcap_capture.c:97)
-        1 : fko_verify_hmac (fko_hmac.c:54)
+        1 : pcap_capture (pcap_capture.c:105)

lib/fko_hmac.c

@@ -61,7 +61,10 @@ int fko_verify_hmac(fko_ctx_t ctx,
     */
     tbuf = strndup(ctx->encrypted_msg, ctx->encrypted_msg_len - SHA256_B64_LEN);
     if(tbuf == NULL)
+    {
+        free(hmac_digest_from_data);
         return(FKO_ERROR_MEMORY_ALLOCATION);
+    }
 
     free(ctx->encrypted_msg);
 
@@ -75,7 +78,10 @@ int fko_verify_hmac(fko_ctx_t ctx,
         res = add_salted_str(ctx);
 
     if (res != FKO_SUCCESS)
+    {
+        free(hmac_digest_from_data);
         return(res);
+    }
 
     /* Calculate the HMAC from the encrypted data and then
      * compare
@@ -95,6 +101,7 @@ int fko_verify_hmac(fko_ctx_t ctx,
         }
     }
 
+    free(hmac_digest_from_data);
     return(res);
 }