Fix signed/unsigned comparison that was upsetting clang.

bionic/libc/stdio/fread.c:86:27: error: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned int') [-Werror,-Wsign-compare]

Change-Id: Ia7e1e053e0cb13113e8f2eede820be013acbab82
diff --git a/libc/stdio/fread.c b/libc/stdio/fread.c
index 9e5758f..baf62b9 100644
--- a/libc/stdio/fread.c
+++ b/libc/stdio/fread.c
@@ -83,7 +83,7 @@
 		/*
 		 * Copy data out of the buffer.
 		 */
-		size_t buffered_bytes = MIN(fp->_r, total);
+		size_t buffered_bytes = MIN((size_t) fp->_r, total);
 		memcpy(dst, fp->_p, buffered_bytes);
 		fp->_p += buffered_bytes;
 		fp->_r -= buffered_bytes;