Zero-extend instructions when fetching them from memory

...since we no longer rely on their being sign-extended.

riscv/mmu.h

@@ -319,15 +319,15 @@ public:
     int length = insn_length(insn);
 
     if (likely(length == 4)) {
-      insn |= (insn_bits_t)from_le(*(const int16_t*)translate_insn_addr_to_host(addr + 2)) << 16;
+      insn |= (insn_bits_t)from_le(*(const uint16_t*)translate_insn_addr_to_host(addr + 2)) << 16;
     } else if (length == 2) {
-      insn = (int16_t)insn;
+      // entire instruction already fetched
     } else if (length == 6) {
-      insn |= (insn_bits_t)from_le(*(const int16_t*)translate_insn_addr_to_host(addr + 4)) << 32;
+      insn |= (insn_bits_t)from_le(*(const uint16_t*)translate_insn_addr_to_host(addr + 4)) << 32;
       insn |= (insn_bits_t)from_le(*(const uint16_t*)translate_insn_addr_to_host(addr + 2)) << 16;
     } else {
       static_assert(sizeof(insn_bits_t) == 8, "insn_bits_t must be uint64_t");
-      insn |= (insn_bits_t)from_le(*(const int16_t*)translate_insn_addr_to_host(addr + 6)) << 48;
+      insn |= (insn_bits_t)from_le(*(const uint16_t*)translate_insn_addr_to_host(addr + 6)) << 48;
       insn |= (insn_bits_t)from_le(*(const uint16_t*)translate_insn_addr_to_host(addr + 4)) << 32;
       insn |= (insn_bits_t)from_le(*(const uint16_t*)translate_insn_addr_to_host(addr + 2)) << 16;
     }