app/testpmd: fix dereference before null check

[ upstream commit f924a8d3bb838bffbaf2c2d47bada952e45b475f ]

Assign 'left' variable only after null check on 'size'
as function returns if 'size' is null.

Coverity issue: 374381
Fixes: 169a9fe ("app/testpmd: fix hex string parser support for flow API")

Signed-off-by: Sean Morrissey <sean.morrissey@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>

app/test-pmd/cmdline_flow.c

@@ -5293,8 +5293,8 @@ parse_string(struct context *ctx, const struct token *token,
 static int
 parse_hex_string(const char *src, uint8_t *dst, uint32_t *size)
 {
-	uint32_t left = *size;
 	const uint8_t *head = dst;
+	uint32_t left;
 
 	/* Check input parameters */
 	if ((src == NULL) ||
@@ -5303,6 +5303,8 @@ parse_hex_string(const char *src, uint8_t *dst, uint32_t *size)
 		(*size == 0))
 		return -1;
 
+	left = *size;
+
 	/* Convert chars to bytes */
 	while (left) {
 		char tmp[3], *end = tmp;