contrib/elf2dmp: add kernel start address checking

Before this patch, if elf2dmp failed to find NT kernel PE magic in
allowed virtual address range, then it assumes NULL as NT kernel
address and cause segfault.

This patch fix the problem described above by checking NT kernel address
before futher processing.

Signed-off-by: Viktor Prutyanov <viktor.prutyanov@phystech.edu>
Message-Id: <20190219211936.6466-1-viktor.prutyanov@phystech.edu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

contrib/elf2dmp/main.c

@@ -524,6 +524,12 @@ int main(int argc, char *argv[])
         }
     }
 
+    if (!nt_start_addr) {
+        eprintf("Failed to find NT kernel image\n");
+        err = 1;
+        goto out_ps;
+    }
+
     printf("KernBase = 0x%016"PRIx64", signature is \'%.2s\'\n", KernBase,
             (char *)nt_start_addr);