bug#44336: 27.1; Calculator doesn't recognize negative exponent in exponent notation

[Nicholas Harrison]

Steps to reproduce:
`M-x calculator RET`
`2e-3 RET`

Expected behavior:
Shows the value 0.002

Actual behavior:
Shows the value 2

Other notes:
Any subsequent calculations use the incorrect value of 2. Positive
exponents, such as 2e3, work as expected. Negative exponents do work properly in
the other calculator mode reached through `M-x calc RET`.

--System Info--
In GNU Emacs 27.1 (build 1, x86_64-w64-mingw32)
 of 2020-08-21 built on CIRROCUMULUS
Repository revision: 86d8d76aa36037184db0b2897c434cdaab1a9ae8
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.19041
System Description: Microsoft Windows 10 Home (v10.0.2004.19041.572)

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