vpx_ports/mem_ops.h: cast the lhs of bitwise shifts of 24.

C does not allow for shifting into the sign bit of a signed
integer, and the two instances here become signed ints via
promotion. Explcitly cast them to unsigned MEM_VALUE_T to
avoid the problem.

BUG=https://bugs.chromium.org/p/chromium/issues/detail?id=614648

Change-Id: I51165361a8c6cbb5c378cf7e4e0f4b80b3ad9a6e
diff --git a/vpx_ports/mem_ops.h b/vpx_ports/mem_ops.h
index 80c034e..620df31 100644
--- a/vpx_ports/mem_ops.h
+++ b/vpx_ports/mem_ops.h
@@ -89,7 +89,7 @@
   unsigned MEM_VALUE_T  val;
   const MAU_T          *mem = (const MAU_T *)vmem;
 
-  val = ((unsigned int)mem[0]) << 24;
+  val = ((unsigned MEM_VALUE_T)mem[0]) << 24;
   val |= mem[1] << 16;
   val |= mem[2] << 8;
   val |= mem[3];
@@ -125,7 +125,7 @@
   unsigned MEM_VALUE_T  val;
   const MAU_T          *mem = (const MAU_T *)vmem;
 
-  val = mem[3] << 24;
+  val = ((unsigned MEM_VALUE_T)mem[3]) << 24;
   val |= mem[2] << 16;
   val |= mem[1] << 8;
   val |= mem[0];