Prevent OOB in SRP base64 code.

Change size comparison from > (GT) to >= (GTE) to ensure an additional
byte of output buffer, to prevent OOB reads/writes later in the function
Reject input strings larger than 2GB
Detect invalid output buffer size and return early

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2672)

crypto/srp/srp_vfy.c

@@ -36,10 +36,13 @@ static int t_fromb64(unsigned char *a, size_t alen, const char *src)
     int i, j;
     int size;
 
+    if (alen == 0 || alen > INT_MAX)
+        return -1;
+
     while (*src && (*src == ' ' || *src == '\t' || *src == '\n'))
         ++src;
     size = strlen(src);
-    if (alen > INT_MAX || size > (int)alen)
+    if (size < 0 || size >= (int)alen)
         return -1;
 
     i = 0;
@@ -77,7 +80,7 @@ static int t_fromb64(unsigned char *a, size_t alen, const char *src)
         if (--i < 0)
             break;
     }
-    while (a[j] == 0 && j <= size)
+    while (j <= size && a[j] == 0)
         ++j;
     i = 0;
     while (j <= size)