Fix division-by-zero error in to_char() with 'EEEE' format.

This fixes a long-standing bug when using to_char() to format a
numeric value in scientific notation -- if the value's exponent is
less than -NUMERIC_MAX_DISPLAY_SCALE-1 (-1001), it produced a
division-by-zero error.

The reason for this error was that get_str_from_var_sci() divides its
input by 10^exp, which it produced using power_var_int(). However, the
underflow test in power_var_int() causes it to return zero if the
result scale is too small. That's not a problem for power_var_int()'s
only other caller, power_var(), since that limits the rscale to 1000,
but in get_str_from_var_sci() the exponent can be much smaller,
requiring a much larger rscale. Fix by introducing a new function to
compute 10^exp directly, with no rscale limit. This also allows 10^exp
to be computed more efficiently, without any numeric multiplication,
division or rounding.

Discussion: https://postgr.es/m/CAEZATCWhojfH4whaqgUKBe8D5jNHB8ytzemL-PnRx+KCTyMXmg@mail.gmail.com

src/test/regress/expected/numeric.out

@@ -1794,6 +1794,38 @@ FROM v;
         NaN |  #.####### |  #.####### |  #.#######
 (7 rows)
 
+WITH v(exp) AS
+  (VALUES(-16379),(-16378),(-1234),(-789),(-45),(-5),(-4),(-3),(-2),(-1),(0),
+         (1),(2),(3),(4),(5),(38),(275),(2345),(45678),(131070),(131071))
+SELECT exp,
+  to_char(('1.2345e'||exp)::numeric, '9.999EEEE') as numeric
+FROM v;
+  exp   |    numeric     
+--------+----------------
+ -16379 |  1.235e-16379
+ -16378 |  1.235e-16378
+  -1234 |  1.235e-1234
+   -789 |  1.235e-789
+    -45 |  1.235e-45
+     -5 |  1.235e-05
+     -4 |  1.235e-04
+     -3 |  1.235e-03
+     -2 |  1.235e-02
+     -1 |  1.235e-01
+      0 |  1.235e+00
+      1 |  1.235e+01
+      2 |  1.235e+02
+      3 |  1.235e+03
+      4 |  1.235e+04
+      5 |  1.235e+05
+     38 |  1.235e+38
+    275 |  1.235e+275
+   2345 |  1.235e+2345
+  45678 |  1.235e+45678
+ 131070 |  1.235e+131070
+ 131071 |  1.235e+131071
+(22 rows)
+
 WITH v(val) AS
   (VALUES('0'::numeric),('-4.2'),('4.2e9'),('1.2e-5'),('inf'),('-inf'),('nan'))
 SELECT val,

src/test/regress/sql/numeric.sql

@@ -939,6 +939,13 @@ SELECT val,
   to_char(val::float4, '9.999EEEE') as float4
 FROM v;
 
+WITH v(exp) AS
+  (VALUES(-16379),(-16378),(-1234),(-789),(-45),(-5),(-4),(-3),(-2),(-1),(0),
+         (1),(2),(3),(4),(5),(38),(275),(2345),(45678),(131070),(131071))
+SELECT exp,
+  to_char(('1.2345e'||exp)::numeric, '9.999EEEE') as numeric
+FROM v;
+
 WITH v(val) AS
   (VALUES('0'::numeric),('-4.2'),('4.2e9'),('1.2e-5'),('inf'),('-inf'),('nan'))
 SELECT val,