Fix a memory leak in circuit_build_times_parse_state

Thanks weasel for noticing.

changes/bug1831

@@ -0,0 +1,3 @@
+  o Minor bugfixes
+    - Fix a memory leak in the error case of circuit_build_times_parse_state().
+      Bugfix on 0.2.2.14-alpha; fixes bug 1831 partially.

src/or/circuitbuild.c

@@ -692,7 +692,8 @@ circuit_build_times_parse_state(circuit_build_times_t *cbt,
             "Corrupt state file? Build times count mismatch. "
             "Read %d times, but file says %d", loaded_cnt,
             state->TotalBuildTimes);
-    *msg = tor_strdup("Build times count mismatch.");
+    if (!*msg)
+      *msg = tor_strdup("Build times count mismatch.");
     circuit_build_times_reset(cbt);
     tor_free(loaded_times);
     return -1;
@@ -716,7 +717,8 @@ circuit_build_times_parse_state(circuit_build_times_t *cbt,
             "Corrupt state file? Shuffled build times mismatch. "
             "Read %d times, but file says %d", tot_values,
             state->TotalBuildTimes);
-    *msg = tor_strdup("Build times count mismatch.");
+    if (!*msg)
+      *msg = tor_strdup("Build times count mismatch.");
     circuit_build_times_reset(cbt);
     tor_free(loaded_times);
     return -1;