Fix floating point exception in phys/module_mp_radar.F when debugging with -D

[jackbartlett20]

Prevent floating point exception in phys/module_mp_radar.F when debugging with -D related to new variables in UDM scheme.

TYPE: bug fix

KEYWORDS: debug, radar, microphysics, UDM, -D

SOURCE: Jack Bartlett (Imperial College London)

DESCRIPTION OF CHANGES:
Problem:
Lines 225 and 226 in phys/module_mp_radar.F trigger a floating point exception when debugging with option -D and using any microphysics scheme which calls radar_init except the UDM scheme because xam_h and xbm_h are not initialised in any other microphysics scheme.

Solution:
Initialise xam_h and xbm_h at 0 and check if they have been given a value greater than 0 before finding their reciprocals.

ISSUE: For use when this PR closes an issue.
Fixes #2248

LIST OF MODIFIED FILES: list of changed files (use git diff --name-status master to get formatted list)
phys/module_mp_radar.F

TESTS CONDUCTED:
Only tested by trying mp_physics=8 with debugging option -D and the new fixes (which previously triggered a fpe).

[weiwangncar]

The regression test results:

Test Type              | Expected  | Received |  Failed
= = = = = = = = = = = = = = = = = = = = = = = =  = = = =
Number of Tests        : 23           24
Number of Builds       : 60           57
Number of Simulations  : 158           150        0
Number of Comparisons  : 95           86        0

Failed Simulations are: 
None
Which comparisons are not bit-for-bit: 
None