C front-end: clarify string initialiser control flow

[tautschnig]

Moving a special case out of a nested if to make the early return more
obvious. This makes it easier to understand where a zero_initializer may
need to be handled.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5302 (03bec50) into develop (bf62883) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5302   +/-   ##
========================================
  Coverage    68.52%   68.52%           
========================================
  Files         1187     1187           
  Lines        98265    98265           
========================================
  Hits         67339    67339           
  Misses       30926    30926           

Flag	Coverage Δ
cproversmt2	42.96% <54.54%> (ø)
regression	65.68% <100.00%> (ø)
unit	32.26% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_initializer.cpp	75.76% <100.00%> (ø)

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[martin-cs]

Seems reasonable.

[chrisr-diffblue]

Thanks - yes, that's clearer :-)