[freetype2] Avoid undefined behavior

Per ISO/IEC 9899:

  If an argument to a function has an invalid value (such as a value
  outside the domain of the function, or a pointer outside the address
  space of the program, or a null pointer, orapointer to non-modifiable
  storage when the corresponding parameter is not const-qualified) or a
  type (after promotion) not expected by a function with variable number
  of arguments, the behavior is undefined. If a function argument is
  described as being an array, the pointer actually passed to the
  function shall have a value such that all address computations and
  accesses to objects (that would be valid if the pointer did point to
  the first element of such an array) are in fact valid.

Per IEEE Std 1003.1:

  size_t fread(void *restrict ptr, size_t size, size_t nitems,
         FILE *restrict stream);

  The fread() function shall read into the array pointed to by ptr up to
  nitems elements whose size is specified by size in bytes, from the
  stream pointed to by stream.

Since the first argument to `fread` is described as being an array, its
behavior is undefined when that argument is a null pointer.

Per the documentation on `ft_ansi_stream_io`:

  If `count' is zero (this is, the function is used for seeking), a
  non-zero return value indicates an error.

Thus the intent is clear, and the call to fread can be skipped, avoiding

Bug: 42500
Change-Id: Id8139730500015b1f3de83aaddcc984ddad4cd60
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/freetype2/+/809648
Reviewed-by: Adam Barth <abarth@google.com>
diff --git a/src/base/ftsystem.c b/src/base/ftsystem.c
index 3013cbd..877bf69 100644
--- a/src/base/ftsystem.c
+++ b/src/base/ftsystem.c
@@ -227,6 +227,9 @@
     if ( stream->pos != offset )
       ft_fseek( file, (long)offset, SEEK_SET );
+    /* avoid calling fread with buffer=NULL and count=0, which is UB. */
+    if ( !count )
+      return 0;
     return (unsigned long)ft_fread( buffer, 1, count, file );