Infinite loop in num_pow

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The following input demonstrates an infinite loop:

1**-1

The problem was introduced in 06c4933. Since -1 >> 1 == -1, exp never reaches zero:

mruby/src/numeric.c

Lines 72 to 73 in ef11584

	exp >>= 1;
	if (exp == 0) break;

This issue was reported by https://hackerone.com/shikchen