target/mips: Handle lock_user() failure in UHI_plog semihosting call

Coverity notes that we forgot to check the error return from
lock_user() in one place in the handling of the UHI_plog semihosting
call.  Add the missing error handling.

report_fault() is rather brutal in that it will call abort(), but
this is the same error-handling used in the rest of this file.

Resolves: Coverity CID 1490684
Fixes: ea4210600db3c5 ("target/mips: Avoid qemu_semihosting_log_out for UHI_plog")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220719191737.384744-1-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

target/mips/tcg/sysemu/mips-semi.c

@@ -321,6 +321,9 @@ void mips_semihosting(CPUMIPSState *env)
             if (use_gdb_syscalls()) {
                 addr = gpr[29] - str->len;
                 p = lock_user(VERIFY_WRITE, addr, str->len, 0);
+                if (!p) {
+                    report_fault(env);
+                }
                 memcpy(p, str->str, str->len);
                 unlock_user(p, addr, str->len);
                 semihost_sys_write(cs, uhi_cb, 2, addr, str->len);