s390: Fix FunctionBodyDecoderTest to compile with gcc4.8.5

GCC4.8.5 on s390 emits warning "array subscript is above array bounds"
for line "code[pos + 1] = kLocalVoid;". The warning seems to be
correct because code[sizeof(code)] should be out of bounds.
I'm suggesting to run the loop till "sizeof(code) - 1" which GCC(4.8.5)
agrees with. Although this means the last byte is missed, but it should
be safe to do since the last few bytes are "0xb" (kExprEnd) and the
offending statement is only run when byte=kExprBlock.

R=titzer@chromium.org, mstarzinger@chromium.org, bradnelson@chromium.org
BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2619063002
Cr-Commit-Position: refs/heads/master@{#42145}

test/unittests/wasm/function-body-decoder-unittest.cc

@@ -1857,7 +1857,7 @@ TEST_F(FunctionBodyDecoderTest, BreakNesting_6_levels) {
 
       int depth = 6;
       int m = mask;
-      for (size_t pos = 0; pos < sizeof(code); pos++) {
+      for (size_t pos = 0; pos < sizeof(code) - 1; pos++) {
         if (code[pos] != kExprBlock) continue;
         if (m & 1) {
           code[pos] = kExprLoop;