x86: work around gcc 3.4.x bug

Simon Horman reported that gcc-3.4.x crashes when compiling
pgd_prepopulate_pmd() when PREALLOCATED_PMDS == 0 and CONFIG_DEBUG_INFO
is enabled.

Adding an extra check for PREALLOCATED_PMDS == 0 [which is compiled out
by gcc] seems to avoid the problem.
Reported-by: NSimon Horman <horms@verge.net.au>
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: NSimon Horman <horms@verge.net.au>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/mm/pgtable.c

@@ -207,6 +207,9 @@ static void pgd_prepopulate_pmd(struct mm_struct *mm, pgd_t *pgd, pmd_t *pmds[])
 	unsigned long addr;
 	int i;
 
+	if (PREALLOCATED_PMDS == 0) /* Work around gcc-3.4.x bug */
+		return;
+
 	pud = pud_offset(pgd, 0);
 
  	for (addr = i = 0; i < PREALLOCATED_PMDS;