cpu/x86/mtrr/mtrr.c:Avoid static scan false positive

Static scan does not know the contents of the fixed MTRR descriptor, so
it has no way to eval the result for variable num_ranges. If num_ranges
is less or equal to 0, the for loop will not be entered, and the values
of fixed_msrs will not be set. Asserting that num_ranges is greater than
0 ensures the loop enters at least once.

BUG=b:112253891
TEST=build grunt

Change-Id: Ieec0ac432c745bde4b1700539c266625da6cfd77
Signed-off-by: Richard Spiegel <richard.spiegel@silverbackltd.com>
Reviewed-on: https://review.coreboot.org/c/31527
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

src/cpu/x86/mtrr/mtrr.c

@@ -36,6 +36,7 @@
 #include <arch/acpi.h>
 #include <memrange.h>
 #include <cpu/amd/mtrr.h>
+#include <assert.h>
 #if IS_ENABLED(CONFIG_X86_AMD_FIXED_MTRRS)
 #define MTRR_FIXED_WRBACK_BITS (MTRR_READ_MEM | MTRR_WRITE_MEM)
 #else
@@ -331,6 +332,7 @@ static void commit_fixed_mtrrs(void)
 
 		desc = &fixed_mtrr_desc[i];
 		num_ranges = (desc->end - desc->begin) / desc->step;
+		ASSERT(num_ranges > 0);
 		for (j = 0; j < num_ranges; j += RANGES_PER_FIXED_MTRR) {
 			msr_index[msr_num] = desc->msr_index_base +
 				(j / RANGES_PER_FIXED_MTRR);