Slightly tidy BIO_C_SET_FILENAME logic

We could just use the string literal as-is.

Change-Id: I2efe01fd9b020db1bb086001407bcf7fa8487551
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/61045
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>

crypto/bio/file.c

diff --git a/crypto/bio/file.c b/crypto/bio/file.c
index 8ba9c54..d0281c8 100644
--- a/crypto/bio/file.c
+++ b/crypto/bio/file.c
@@ -172,7 +172,6 @@
   long ret = 1;
   FILE *fp = (FILE *)b->ptr;
   FILE **fpp;
-  char p[4];
 
   switch (cmd) {
     case BIO_CTRL_RESET:
@@ -197,27 +196,28 @@
     case BIO_C_SET_FILENAME:
       file_free(b);
       b->shutdown = (int)num & BIO_CLOSE;
+      const char *mode;
       if (num & BIO_FP_APPEND) {
         if (num & BIO_FP_READ) {
-          OPENSSL_strlcpy(p, "a+", sizeof(p));
+          mode = "a+";
         } else {
-          OPENSSL_strlcpy(p, "a", sizeof(p));
+          mode = "a";
         }
       } else if ((num & BIO_FP_READ) && (num & BIO_FP_WRITE)) {
-        OPENSSL_strlcpy(p, "r+", sizeof(p));
+        mode = "r+";
       } else if (num & BIO_FP_WRITE) {
-        OPENSSL_strlcpy(p, "w", sizeof(p));
+        mode = "w";
       } else if (num & BIO_FP_READ) {
-        OPENSSL_strlcpy(p, "r", sizeof(p));
+        mode = "r";
       } else {
         OPENSSL_PUT_ERROR(BIO, BIO_R_BAD_FOPEN_MODE);
         ret = 0;
         break;
       }
-      fp = fopen(ptr, p);
+      fp = fopen(ptr, mode);
       if (fp == NULL) {
         OPENSSL_PUT_SYSTEM_ERROR();
-        ERR_add_error_data(5, "fopen('", ptr, "','", p, "')");
+        ERR_add_error_data(5, "fopen('", ptr, "','", mode, "')");
         OPENSSL_PUT_ERROR(BIO, ERR_R_SYS_LIB);
         ret = 0;
         break;