psql: Fix incorrect equality comparison

Commit 1a759c8327 contained an incorrect equality comparison
which was discovered by Coverity.

Reported-by: Ranier Vilela <ranier.vf@gmail.com>
Discussion: https://postgr.es/m/CAEudQApfAWzLo+oSuy2byXktdr7R8KJC_ACT5VV8fontrL35Pw@mail.gmail.com

src/bin/psql/variables.c

@@ -234,7 +234,7 @@ ParseVariableDouble(const char *value, const char *name, double *result, double
 	 * too close to zero to have full precision, by checking for zero or real
 	 * out-of-range values.
 	 */
-	else if ((errno = ERANGE) &&
+	else if ((errno == ERANGE) &&
 			 (dblval == 0.0 || dblval >= HUGE_VAL || dblval <= -HUGE_VAL))
 	{
 		if (name)