JIT: initialize emitCurIG at declaration

[AndyAyersMS]

The RISC-V emitter has cost-estimation calls into emitLoadImmediate from gtSetEvalOrder that hit the new emitGeneratingPrologOrFuncletProlog check before codegen runs emitter::Init(), reading a poisoned emitCurIG and crashing (SIGSEGV in emitIGisInProlog(ig=0xdddddddddddddddd)).

Fixes #130145.

Verified locally by reproducing the CI failure with an x64-host RISC-V cross-JIT on ilc @ilc.ilc.rsp: prior to this change, exits 139 with a coredump whose stack is emitIGisInProlog(0xdddd...) <- emitGeneratingPrologOrFuncletProlog <- emitLoadImmediate<false> <- gtSetEvalOrder <- fgFindOperOrder <- compCompile while JITing System.DateTimeOffset:ToString(). With this change, ilc produces the RISC-V ilc.o and exits 0.

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Pull request overview

This PR makes the JIT emitter’s current instruction group pointer (emitCurIG) default-initialize to nullptr so that helper queries like emitGeneratingPrologOrFuncletProlog() behave safely before emitter::Init() runs.

Changes:

• Initialize emitter::emitCurIG at declaration (= nullptr) to avoid reading an uninitialized/poisoned pointer prior to Init().
• Ensures early-phase callers that consult prolog/funclet state (via emitGeneratingPrologOrFuncletProlog()) return false instead of dereferencing garbage.

[AndyAyersMS]

@dotnet/jit-contrib PTAL -- tiny change