[lib] Validate buffers when ZSTD_c_stable{In,Out}Buffer is set

Adds the validation of the input/output buffers only. They are still
unused.

lib/common/zstd_errors.h

@@ -77,6 +77,7 @@ typedef enum {
   ZSTD_error_frameIndex_tooLarge = 100,
   ZSTD_error_seekableIO          = 102,
   ZSTD_error_dstBuffer_wrong     = 104,
+  ZSTD_error_srcBuffer_wrong     = 105,
   ZSTD_error_maxCode = 120  /* never EVER use this value directly, it can change in future versions! Use ZSTD_isError() instead */
 } ZSTD_ErrorCode;