Use C99 for size_t loops.

This was done just by grepping for 'size_t i;' and 'size_t j;'. I left
everything in crypto/x509 and friends alone.

There's some instances in gcm.c that are non-trivial and pulled into a
separate CL for ease of review.

Change-Id: I6515804e3097f7e90855f1e7610868ee87117223
Reviewed-on: https://boringssl-review.googlesource.com/10801
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/modes/gcm_test.c

diff --git a/crypto/modes/gcm_test.c b/crypto/modes/gcm_test.c
index 19c295b..6f9d528 100644
--- a/crypto/modes/gcm_test.c
+++ b/crypto/modes/gcm_test.c
@@ -257,9 +257,6 @@
 
 static int decode_hex(uint8_t **out, size_t *out_len, const char *in,
                       unsigned test_num, const char *description) {
-  uint8_t *buf = NULL;
-  size_t i;
-
   if (in == NULL) {
     *out = NULL;
     *out_len = 0;
@@ -269,16 +266,16 @@
   size_t len = strlen(in);
   if (len & 1) {
     fprintf(stderr, "%u: Odd-length %s input.\n", test_num, description);
-    goto err;
+    return 0;
   }
 
-  buf = OPENSSL_malloc(len / 2);
+  uint8_t *buf = OPENSSL_malloc(len / 2);
   if (buf == NULL) {
     fprintf(stderr, "%u: malloc failure.\n", test_num);
     goto err;
   }
 
-  for (i = 0; i < len; i += 2) {
+  for (size_t i = 0; i < len; i += 2) {
     uint8_t v, v2;
     if (!from_hex(&v, in[i]) ||
         !from_hex(&v2, in[i+1])) {