PVS-Studio warning V610 for MesaLib

#define MAX_VARYING 32

typedef enum
{
  ....
  FRAG_ATTRIB_VAR0 = 16,
  FRAG_ATTRIB_MAX = (FRAG_ATTRIB_VAR0 + MAX_VARYING)
} gl_frag_attrib;

static inline GLuint
clip_span( struct gl_context *ctx, SWspan *span )
{
  ....
  for (i = 0; i < FRAG_ATTRIB_MAX; i++) {
    if (span->interpMask & (1 << i)) {
  ....
}

This suspicious code was found in MesaLib project by PVS-Studio static code analyzer.
Warning message is:
V610 Undefined behavior. Check the shift operator '<<. The right operand ('i' = [0..47]) is greater than or equal to the length in bits of the promoted left operand. s_span.c 768

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