Fix compilation error on gcc 4.8.5 on s390x

This fixes "error: array subscript is below array bounds" error
message generated by gcc 4.8.5

R=mvstanton@chromium.org, bmeurer@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2299763002
Cr-Commit-Position: refs/heads/master@{#39086}

src/base/ieee754.cc

@@ -609,7 +609,7 @@ recompute:
     j = 0;
     for (i = jz - 1; i >= jk; i--) j |= iq[i];
     if (j == 0) { /* need recomputation */
-      for (k = 1; iq[jk - k] == 0; k++) {
+      for (k = 1; jk >= k && iq[jk - k] == 0; k++) {
         /* k = no. of terms needed */
       }