Fix uninitialized variable in x509_crt

This patch fixes an issue we encountered with more stringent compiler
warnings.  The signature_is_good variable has a possibility of being
used uninitialized.  This patch moves the use of the variable to a
place where it cannot be used while uninitialized.

Signed-off-by: Andy Gross <andy.gross@linaro.org>
diff --git a/ChangeLog b/ChangeLog
index a782578..fea2bc2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -23,6 +23,9 @@
    * Fix propagation of restart contexts in restartable EC operations.
      This could previously lead to segmentation faults in builds using an
      address-sanitizer and enabling but not using MBEDTLS_ECP_RESTARTABLE.
+   * Improve code clarity in x509_crt module, removing false-positive
+     uninitialized variable warnings on some recent toolchains (GCC8, etc).
+     Discovered and fixed by Andy Gross (Linaro), #2392.
 
 Changes
    * Make it easier to define MBEDTLS_PARAM_FAILED as assert (which config.h