[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/error_private.c

@@ -48,6 +48,7 @@ const char* ERR_getErrorString(ERR_enum code)
     case PREFIX(frameIndex_tooLarge): return "Frame index is too large";
     case PREFIX(seekableIO): return "An I/O error occurred when reading/seeking";
     case PREFIX(dstBuffer_wrong): return "Destination buffer is wrong";
+    case PREFIX(srcBuffer_wrong): return "Source buffer is wrong";
     case PREFIX(maxCode):
     default: return notErrorCode;
     }

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;