xen/arm64: Make show_registers working even if current is not correctly set

The commit 8ab802c "xen/arm: Restricted access to IFSR32_EL2 and FPEXC32_EL2"
introduced the use of the variable current in show_registers.

The variable is not correctly set until Xen has finished to boot.
Therefore any call to show_registers will result to crash Xen.

As the register IFSR32_EL2 is only printed when the registers represent
a guest view, Xen can read the value only for this case.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

xen/arch/arm/traps.c

@@ -791,7 +791,7 @@ void show_registers(struct cpu_user_regs *regs)
 #else
     ctxt.far = READ_SYSREG(FAR_EL1);
     ctxt.esr_el1 = READ_SYSREG(ESR_EL1);
-    if ( is_32bit_domain(current->domain) )
+    if ( guest_mode(regs) && is_32bit_domain(current->domain) )
         ctxt.ifsr32_el2 = READ_SYSREG(IFSR32_EL2);
 #endif
     ctxt.vttbr_el2 = READ_SYSREG64(VTTBR_EL2);