Fix numeric_power() when the exponent is INT_MIN.

In power_var_int(), the computation of the number of significant
digits to use in the computation used log(Abs(exp)), which isn't safe
because Abs(exp) returns INT_MIN when exp is INT_MIN. Use fabs()
instead of Abs(), so that the exponent is cast to a double before the
absolute value is taken.

Back-patch to 9.6, where this was introduced (by 7d9a4737c2).

Discussion: https://postgr.es/m/CAEZATCVd6pMkz=BrZEgBKyqqJrt2xghr=fNc8+Z=5xC6cgWrWA@mail.gmail.com

src/backend/utils/adt/numeric.c

@@ -8433,7 +8433,7 @@ power_var_int(const NumericVar *base, int exp, NumericVar *result, int rscale)
 	 * to around log10(abs(exp)) digits, so work with this many extra digits
 	 * of precision (plus a few more for good measure).
 	 */
-	sig_digits += (int) log(Abs(exp)) + 8;
+	sig_digits += (int) log(fabs(exp)) + 8;
 
 	/*
 	 * Now we can proceed with the multiplications.

src/test/regress/expected/numeric.out

@@ -1623,6 +1623,12 @@ select 0.12 ^ (-20);
  2608405330458882702.5529619561355838
 (1 row)
 
+select 1.000000000123 ^ (-2147483648);
+      ?column?      
+--------------------
+ 0.7678656556403084
+(1 row)
+
 -- cases that used to error out
 select 0.12 ^ (-25);
                  ?column?                  

src/test/regress/sql/numeric.sql

@@ -896,6 +896,7 @@ select 3.789 ^ 21;
 select 3.789 ^ 35;
 select 1.2 ^ 345;
 select 0.12 ^ (-20);
+select 1.000000000123 ^ (-2147483648);
 
 -- cases that used to error out
 select 0.12 ^ (-25);