programs/ssl: Fix incorrect EOF check in ssl_context_info.c

In `read_next_b64_code()`, the result of fgetc() is stored into a char,
but later compared against EOF, which is generally -1.  On platforms
where char is unsigned, this generates a compiler warning/error that the
comparison will never be true (causing a build failure).  The value will
never match, with the function ultimately bailing with a "Too many bad
symbols are detected" error.

On platforms with signed char, EOF is detected, but a file containing a
0xFF character will causes a premature end of file exit of the loop.

Fix this by changing the result to an int.

Fixes #3794.

Signed-off-by: Nayna Jain <[email protected]>
Signed-off-by: David Brown <[email protected]>

ChangeLog.d/bugfix_3794.txt

@@ -0,0 +1,4 @@
+Bugfix
+  * Fix handling of EOF against 0xff bytes and on platforms with
+    unsigned chars.  Fixes a build failure on platforms where char is
+    unsigned.  Fixes #3794.

programs/ssl/ssl_context_info.c

@@ -377,13 +377,13 @@ size_t read_next_b64_code( uint8_t **b64, size_t *max_len )
     int valid_balance = 0;  /* balance between valid and invalid characters */
     size_t len = 0;
     char pad = 0;
-    char c = 0;
+    int c = 0;
 
     while( EOF != c )
     {
         char c_valid = 0;
 
-        c = (char) fgetc( b64_file );
+        c = fgetc( b64_file );
 
         if( pad > 0 )
         {