cpu: x64: gemm: fix sgemv thread checker

Author: aaraujom

src/cpu/x64/gemm/gemv_driver.cpp

@@ -257,8 +257,8 @@ static inline int thread_checker(
             if (bandt == 0) {
                 return 1;
             } else {
-                return nstl::min(
-                        nstl::max(n * m / 2 * MN_MIN_N, dim_t(1)), dim_t(nthr));
+                return nstl::min(nstl::max(n * m / (2 * MN_MIN_N), dim_t(1)),
+                        dim_t(nthr));
             }
         }
     } else {
@@ -290,6 +290,12 @@ static inline void decompose_vector(const dim_t m, const dim_t nthr,
     dim_t loffset = 0;
     dim_t lsize = 0;
 
+    if (ithr >= nthr) {
+        *offset = loffset;
+        *size = lsize;
+        return;
+    }
+
     if (addr == NULL) {
         dim_t xthr = m % nthr;
         dim_t width = m / nthr;